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.