Federal authorities have openly expressed concern that Americans who were recruited to join an Al Qaeda-linked group in Somalia could easily return to the United States thanks to their U.S. passports. But privately law enforcement officials are also worried about another possible scenario: Fighters from the group could be smuggled into the United States to launch attacks.

(Pictured: Salah Osman Ahmed, a Minneapolis man who joined an Al Qaeda-linked group in Somalia before returning to the United States in April 2008)

"It's certainly something that quite plainly is definitely on the radar," said one official with U.S. Customs and Border Protection, which oversees border security operations. "We're alert to that and doing our part to make sure that we address that threat."

CBP's part includes keeping Border Patrol agents, customs officers and other immigration officials informed of emerging concerns and trends. In February CBP sent an alert -- or "issue paper" -- about Somalia and the Al Qaeda-linked group, known as al-Shabaab, to offices across the country.

"It is feared that U.S. citizens and residents are being called to action in Somalia, as well as to support the illegal migration of Somalis to the United States," said one sentence in the alert, as relayed to FOX News.

Of all the law enforcement agencies, CBP has a particular interest in the issue.

"We are the first line of defense," said CBP spokesman Lloyd Easterling, referring to the more than 22,000 officers at border crossings, airports and marine terminals, and the more than 17,000 Border Patrol agents between those ports of entry. "Many times, our officers are the first to make contact with people arriving into the United States, which allows us a unique opportunity to address these threats immediately."

For the past year, the FBI has been investigating how more than 20 young Americans -- most of them of Somali descent -- were recruited to join al-Shabaab, which is fighting to establish a Muslim state in Somalia and recently pledged its allegiance to Usama bin Laden.

Last month FBI Director Robert Mueller became the first U.S. official to acknowledge that al-Shabaab "would like to undertake operations outside of Somalia." But, he said, it has yet to do so.

In congressional hearings and public briefings over the past several months, discussions about al-Shabaab's possible presence in the United States have focused on how American recruits can "travel rather freely" with their U.S. passports, as one counterterrorism official put it in March.

"We are concerned that if a few Somali American youth could be motivated to engage in such activities overseas, fellow travelers could return to the U.S. and engage in terrorist activities here," Andrew Liepman, the deputy director for intelligence at the National Counterterrorism Center, told lawmakers at a Senate hearing on al-Shabaab.

Officials and lawmakers have never publicly raised the possibility of al-Shabaab sending recruits -- of any nationality -- to enter the United States illegally. The issue, though, has come up in internal conversations, immigration and other law enforcement officials said.

The discussions have not been limited to CBP. Officials within the FBI, the Immigration and Customs Enforcement agency, and the Intelligence and Analysis Office of the Department of Homeland Security, which oversees CBP and ICE, have discussed the possibility that al-Shabaab members could be smuggled into the United States to launch attacks, DHS and FBI officials said.

"[We] and the rest of the U.S. government certainly recognize it as a threat," said an official with ICE, which investigates suspected cases of illegal entry into the United States.

ICE officials wouldn't say whether their agency, like CBP, has issued alerts about Somalia and al-Shabaab, but one ICE official insisted, "I can assure you that our agents are well aware of what nationalities we're concerned about and could be terrorist threats."

For now, the intelligence community believes the threat of al-Shabaab launching attacks inside the United States is low.

There is "no hard information or evidence that has been effectively pursued," Mueller said last month. And attempts to send a foreign national to the United States to launch attacks would likely fail, one source said.

"The likelihood of them being successful, especially with all the attention on them now, is not high," the source said.

In addition, al-Shabaab would likely think twice before directly attacking the United States.

"It's a riskier move on their part, which would create a U.S. response in the Horn of Africa," the source said.

But officials do worry all that could change -- especially if Al Qaeda operatives working with al-Shabaab in Somalia pressure the group to look outside East Africa.

"It is those Al Qaeda elements that we fear will push al-Shabaab members to change their focus," Michael Leiter, the director of the National Counterterrorism Center, told lawmakers last month.

An ICE official offered a similar assessment, saying authorities are watching for any shifts in al-Shabaab's aims.

"We're obviously concerned about what happens in Somalia, some of the radicalization that is occurring there," the official said. "Being as unstable as [Somalia] is, we're greatly concerned about Somalis being smuggled into the U.S."

Officials didn't say how al-Shabaab members might be smuggled into the United States, but they are taking note of how Somalis -- with no ill intentions -- have already managed to enter the country illegally.

The "vast majority have been smuggled across" the southern U.S. border and "are likely walked across by guides," orchestrated by alien smuggling operations throughout South and Central America, according to an ICE official.

Mexico, in particular, has become a pipeline of sorts for illegal immigration between Somalia and the United States.

"The alien smuggling organization understands [Somalis'] desperation and seeks to profit from illegal migrants," CBP's alert in February said, as relayed to FOX News.

In some cases, Somalis have used fraudulent documents, including fake passports, to enter the country through a sanctioned border crossing, but those instances are rare, according to the official.

Asked whether Mexican drug cartels could be involved in smuggling Somalis, the official said there is "at least the thought out there that some of the cartels" could transport Somalis, including associates of al-Shabaab. But, another ICE official said, networks that "could be used to move people who want to do us harm" are "not ideologically based," they're profit-based.

As CBP's alert in February indicated, many Somalis entering the United States illegally are also likely getting help from U.S. citizens and residents.

People inside the United States can contribute funds, provide hideouts and homes, purchase bus or plane tickets, or support the smuggling of Somalis in other ways, a CBP official said.

"It depends largely on the job they're doing and the risks they want to take," the official said.

Over all, ICE officials said, they are not seeing "huge numbers" of Somalis being smuggled into the United States.

What's more, officials and experts on Somalia agree that the vast majority -- if not all -- of Somalis who have entered the country illegally have no intention of harming the United States.

"Most Somalis are just looking for a better life, and we have to be careful not to consider every Somali to be a threat," said Bruno Schiemsky, a Kenya-based consultant specializing in extremist movements in Eastern Africa.

The alert issued by CBP, which said most Somalis encountered by CBP are men between 18 and 29 years old, reflected that thought.

"The continued deterioration of living conditions in Somalia is expected to sustain the illegal migration of Somalis to the United States," the alert said, as relayed to FOX News. "The bleak future of Somalia has caused and will continue to cause refugees to seek a more sustainable existence, which includes risking thousands of dollars and their lives to be smuggled to the United States."

Smuggling, though, is not the only possible option for associates of al-Shabaab to get into the country. Specifically, Americans who are "being called to action in Somalia," as CBP's alert put it, can try to enter the United States legally.

And while Easterling said CBP "regularly refuses entry to people who may pose a threat to the security of our country," sometimes CBP allows even associates of al-Shabaab to enter.

One case in particular could explain why.

Salah Osman Ahmed (pictured), in his mid-20s, was one of the first Americans to be radicalized inside the United States and persuaded to train with al-Shabaab in Somalia.

In December 2007, he flew from Minneapolis to the Netherlands, with a final destination of Somalia. He wanted to fight against Ethiopian troops backing the more secular, transitional government there, according to court documents.

Four months later, in April 2008, he returned to the United States, arriving at an airport in New York. After giving CBP officers the appropriate documents, Ahmed acknowledged to a CBP officer that he had traveled to Somalia.

Ahmed was then detained and questioned by CBP officers, according to his lawyer, Jim Ostgard, and a source with knowledge of the case. Officers also searched "his person" and looked through Ahmed's luggage, Ostgard said.

Ahmed was not on any terrorism watchlist, but his travel to Somalia raised the CBP officers' concern, the source said, suggesting that CBP was already well aware of the possible implications of traveling to Somalia.

Ahmed was eventually released and allowed to board a flight to Minneapolis. A "very brief written report" of Ahmed's "encounter" at the airport was sent to the FBI, according to Ostgard.

A CBP official said that, even in cases when a person is on a terrorism watchlist, CBP would notify the appropriate agencies and only detain the person if needed. Any further investigation would be conducted by the appropriate agency, the official said.

The FBI did follow up in Ahmed's case. Three months after he returned to the United States, Ahmed was interviewed by FBI agents about his time in Somalia and others who traveled there, according to court documents.

In December 2008, FBI agents returned for a second interview, and two months later a grand jury in Minneapolis indicted him on charges of providing material support to terrorists. He wasn't arrested until July, shortly before his case became public. He has since reached a plea agreement with the government.

It's unclear why Ahmed was not arrested immediately, but several Minnesota men who traveled to Somalia had returned to the United States by the time of Ahmed's arrest. They were not arrested immediately so that federal authorities could continue their investigation, a law enforcement source said.

Three other Somali-Americans have since been indicted in connection with the FBI investigation. Two men have pleaded guilty to providing material support to terrorists, and another was recently indicted for lying to FBI agents. He has pleaded not guilty.

Asked why Ahmed decided to return to the United States, Ostgard declined to comment.