(CNN) The State Department is offering $5 million for information that leads to the arrest of an American citizen on the FBI's Most Wanted Terrorist List.

An indictment unsealed in federal court on Monday accuses Jehad Serwan Mostafa, 37, of providing material support to al-Shabaab , which the US has designated a terrorist organization for its activities to undermine the Somali government.

"We believe this defendant is the highest-ranking U.S. citizen fighting overseas with a terrorist organization," US Attorney for the Southern District of California Robert Brewer said in a news release Monday. "Al-Shabaab's reign of terror threatens U.S. national security, our international allies and innocent civilians. Today we seek the public's assistance in capturing Mostafa and disrupting Al-Shabaab."

Mostafa -- also known as "Ahmed Gurey," "Ahmed," "Anwar," "Abu Anwar al Muhajir," and "Abu Abdallah al Muhajir" -- was formerly a resident of San Diego, but is currently believed to be in Somalia, according to San Diego special-agent-in-charge Scott Brunner.

He has been linked to the use of improvised explosive devices in attacks in Somalia, according to the release.

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