In the last decade, Jehad Serwan Mostafa has become what the federal authorities call the “highest-ranking U.S. citizen fighting overseas with a terrorist organization.”
After he left his hometown, San Diego, in 2005 at the age of 23, Mr. Mostafa joined the extremist militant group Al Shabab in its brutal campaign against the Somali government, training Shabab soldiers, organizing the group’s media appearances and taking part in attacks against Somali and African Union forces, the authorities said.
But Mr. Mostafa, 37, who is on the F.B.I.’s list of most wanted terrorists, has eluded capture.
That has increasingly alarmed the federal authorities who, at a news conference on Monday, unsealed a new federal indictment against him and reminded the public about a $5 million reward for information leading to his arrest — two moves they hope will draw more help from the public.
“We must use every tool in our toolbox to disrupt a potential terrorist attack before it happens,” Scott Brunner, the special agent in charge of the F.B.I. field office in San Diego, said at the news conference.
The authorities said they had recently learned that Mr. Mostafa, who they believe is in southern Somalia, had taken up a leadership role with Al Shabab focusing on explosives, though they declined to provide specifics. They also warned that one of Al Shabab’s leaders issued a “call to arms” in November to attack Americans around the world.
“Al Shabab’s reign of terror threatens U.S. national security, our international allies and innocent civilians,” Robert Brewer, the United States attorney in San Diego, said in a statement on Monday. “Today we seek the public’s assistance in capturing Mostafa and disrupting Al Shabab.”
Mr. Mostafa faces one count each of conspiracy to provide material support to terrorists, conspiracy to provide material support to a foreign terrorist organization and providing material support to a foreign terrorist organization.
If convicted, he would face up to 55 years in prison and $750,000 in fines, prosecutors said.
The superseding indictment announced this week — brought by a federal grand jury in January and unsealed on Monday — was the second against Mr. Mostafa. He was previously indicted on similar charges in 2009. The new indictment covers his activities over a greater period of time, Caroline Han, an assistant United States attorney in San Diego, said at the news conference.
The $5 million reward has existed since 2013.
Al Shabab, which has killed thousands in attacks across East Africa, was designated by the United States as a foreign terrorist organization in 2008. In 2012, the group proclaimed allegiance to Al Qaeda.
It carried out the 2013 attack at the Westgate mall, which killed more than 60 people and wounded more than 175 in Nairobi, Kenya. In July, the group claimed responsibility for an attack on a hotel in Mogadishu, Somalia, that left at least 26 people dead.
The authorities declined to provide more specifics on Mr. Mostafa’s specific activities as part of Al Shabab, or say whether he may have been linked to the high-profile attacks in Nairobi or Mogadishu.
Mr. Mostafa was born in Wisconsin and grew up in San Diego, the authorities said. He was raised in the Serra Mesa neighborhood, graduated from high school in the city, attended Mesa College, a community college, and graduated from the University of California, San Diego, in 2004, the authorities said.
It was not clear what motivated Mr. Mostafa to join Al Shabab. He first traveled to Somalia in 2005 and fought against Ethiopian forces there, the authorities said.
In 2011, he appeared at a press event with a Shabab leader and was described as a representative of Al Qaeda, Mr. Brunner said. During the event, he praised Osama bin Laden, Mr. Brunner said.
Sheelagh McNeill contributed research.