Abu Mansour al-Amriki — or Omar Hammami as he was known — appears in a video posted online Tuesday by the group al-Shabaab, which has ties to Al Qaeda and was labeled a terrorist organization by the U.S. government last year. In the new video, al-Shabaab fighters pledge their allegiance to Usama bin Laden.

"I ask Allah to grant victory to the [Muslim fighters] in every land and under every sky," says one fighter in the video, excerpts of which were provided to Fox News by the Washington-based Middle East Media Research Institute (MEMRI). "We ask Allah that the Islamic Nation be liberated from invaders by the next [end of Ramadan] and [an Islamic state] be established by then."

During the 50-minute video, titled "We Are At Your Command, Usama," al-Amriki pops up in scenes of apparent military training. In one scene, he stands in the background – a black, bulging vest strapped to his chest – while at least four people whose faces are hidden by cloth point rifles forward.

Al-Amriki has been in Somalia for several years, but he first became a public figure in October 2007, when Al-Jazeera TV featured him in a report on the "common goal" of Al Qaeda and hard-line militants in Somalia. He kept his face hidden.

He showed his face for the first time in April, when a video posted online featured him purportedly leading a group of al-Shabaab militants in an ambush of "the enemy."

Speaking about one man killed in the fight, he said, "We need more like him, so if you can encourage more of your children and more of your neighbors, anyone around, to send people like him to this jihad, it would be a great asset for us."

His efforts in Somalia spurred the Federal Bureau of Investigation to launch an investigation several years ago. The case was ultimately brought before a grand jury in Mobile, Ala., where he has been indicted on charges of providing material support to terrorists, a source said. It’s unclear when that indictment was filed.

As FOX News first reported three weeks ago, al-Amriki was born Omar Hammami in May 1984, and he grew up outside Mobile, Ala., in the quiet city of Daphne.

He was raised Baptist like his mother, but his Syrian-born father is Muslim, and "some time in high school" Hammami converted to Islam, a woman who went to high school with Hammami told FOX News.

The woman, Shellie Brooks, said Hammami would take time out from classes throughout the day to pray. At the same time, he would frequent the Islamic Society of Mobile, one of the most popular mosques in the Mobile area.

The school's principal, Don Blanchard, described Hammami as "a good kid," and Brooks called him a "very intellectual guy" who was "really well liked" and "had tons of friends."

But, Brooks said, "things changed a bit when he converted because his beliefs changed."

He left high school a year early, forgoing a fourth year at Daphne High School to enroll as a 17-year-old at the University of South Alabama in Mobile.

He chose computer science as his major, but he dropped out within two years.

According to a source within Mobile's Muslim community who described hismelf as an "acquaintance" of Hammami, the college drop-out then went to Toronto, where he married a Somali woman he had met through a mutual friend. The couple had a child together, the source said.

After living in Toronto "for a while," some "angry" individuals there "brainwashed" him and "encouraged him to fight in Somalia," according to the source.

Hammami then left his wife and child for Somalia, the source said.

Another source in the Mobile area said Hammami has not spoken to his family – including his mother, father, sister and ex-wife – in at least four years.

The Royal Canadian Mounted Police declined to comment, and an FBI spokeswoman said he could not confirm the sources' accounts.

Hammami – or al-Amriki – is a small part of the new video posted online by al-Shabaab. The video centers around a ceremony in the Kismaayo district of southern Somalia, during which some Somalis vow obedience to bin Laden by declaring, "We are at your command, oh Usama," according to MEMRI.

U.S. officials have become increasingly concerned that, if al-Shabaab prevails in Somalia, the country could turn into a haven for Al Qaeda and other terrorist groups. Somalia has had no stable government since 1991, when dictator Siad Barre was ousted from power. A newer secular government has had trouble keeping Muslim militants at bay, and in 2006 fighting with al-Shabaab intensified after Western-backed Ethiopian forces invaded Somalia.

The United States and other countries have recently been assisting Somalia's government in the fight against al-Shabaab. Meanwhile, federal law enforcement have been keeping a close on al-Shabaab's moves overseas.

In addition to Hammami's case, for much of the past year the FBI has been looking into how dozens of young men from the Minneapolis area and elsewhere were recruited to train and possibly fight alongside al-Shabaab in Somalia.

In October 2008, 27-year-old college student Shirwa Ahmed of Minneapolis became "the first known American suicide bomber" when he blew himself up in Somalia, killing dozens, according to the FBI.

Since then at least four more men from Minneapolis have been killed in Somalia, according to their families.

Earlier this year a grand jury in Minneapolis indicted at least three men. They have already pleaded guilty to terror-related charges, including providing material support to terrorists.

The indictments said the men traveled to Somalia "so that they could fight jihad" there.