The FBI on Thursday announced it was adding a former taxi driver with Northern ­Virginia ties to its “Most Wanted Terrorists” list, saying the 29-year-old was suspected of providing support to al-Shabab, as well as al-Qaeda, and was thought to have fled to Somalia to join up with a terrorist group.

Liban Haji Mohamed, a naturalized U.S. citizen who was born in Somalia, was charged in a warrant unsealed Thursday in federal district court in Alexandria with providing material support and resources to a designated terrorist organization. At the same time, the FBI announced it was offering a $50,000 reward for information leading to his arrest and conviction and launched a significant online campaignincluding a Somali-language Facebook page — to get the word out that Mohamed is wanted.

According to the FBI, Mohamed lived in Northern Virginia and drove a taxi for years before he departed for East Africa in July 2012.

This is not the family’s first encounter with law enforcement. In 2011, Liban Mohamed’s then-teenage brother, Gulet Mohamed, made national headlines when he was put on a no-fly list and barred from taking a flight home from Kuwait. Gulet Mohamed spent weeks in detention there — allegedly beaten by Kuwaiti authorities and questioned by FBI agents about possible terrorist connections — and still is pursuing a civil lawsuit over the incident. Liban Mohamed submitted an affidavit in that case years ago, describing how he became concerned when he had not heard from his brother and tried to call and e-mail authorities to figure out what had happened. He said when he finally talked to his brother, Gulet Mohamed described being interrogated and tortured.

The FBI said Liban Mohamed was a close associate of Zachary Chesser, an Oakton High School graduate who threatened the creators of the “South Park” TV show over their depiction of the prophet Muhammad and tried unsuccessfully to join al-Shabab in Somalia.

Former Northern Virginia taxi driver Liban Haji Mohamed is on the FBI’s “Most Wanted Terrorists” list, with a $50,000 reward offered for information. (Courtesy of FBI)

Chesser pleaded guilty in 2010 to providing material support to terrorists, communicating threats and soliciting crimes of violence. He was sentenced to 25 years in prison.

Efforts to reach U.S. relatives of Liban Mohamed were unsuccessful on Thursday.

But Gadeir Abbas, an attorney for Gulet Mohamed, said the unsealing of the warrant for Liban Mohamed — and the lack of detail on the basis of the charges — was “questionable” given that there is a hearing scheduled in his client’s civil lawsuit Friday.

“Why didn’t they do this earlier?” he said.

In an online statement, Carl Ghattas, special agent in charge of the Counterterrorism Division at the FBI’s Washington field office, said Liban Mohamed was “a recruiter and radicalizer for al Shabaab” while he was living in Northern Virginia. He said Mohamed “took a prominent role in trying to recruit people and have them train with weapons,” and ultimately decided to fight in Africa.

“It is important for us to locate Mohamed because he has knowledge of the Washington, D.C. area’s infrastructure such as shopping areas, Metro, airports, and government buildings,” Ghattas said in the statement. “This makes him an asset to his terrorist associates who might plot attacks on U.S. soil.”

The FBI said Liban Mohamed speaks English, Somali and ­Arabic, and uses the aliases Abu Ayrow, Shirwa, Shirwac, Qatiluhum and Qatil. He is suspected to have passed through the Ugandan city of Kampala on his way to ­Somalia.

Anyone with information is asked to contact an FBI field office or the nearest American embassy or consulate.