The U.S. State Department is offering a $5 million prize for information leading to the arrest of Jehad Serwan Mostafa, an American citizen on the FBI’s Most Wanted List.

According to U.S. Attorney for the Southern District of California Robert Brewer, Mostafa is believed to be the “highest-ranking U.S. citizen fighting overseas with a terrorist organization.” The thirty-seven year old American is accused of providing material support to the Somali-based terrorist group al-Shabaab.

Born in Wisconsin, Mostafa grew up in San Diego, but then left for Somalia in 2005. It was there that he fought against the Ethiopian intervention in the country and later joined al-Shabaab, which is an al-Qaeda affiliate that seeks to enforce strict sharia law. The group was designated a terrorist organization by the U.S. government in 2008 and has been responsible for numerous attacks in east Africa.

Though Mostafa has been under indictment since 2009, the recent unsealment of an indictment in federal court reveals that this has widened in scope to include allegations brought towards him up to 2017. According to the newly revealed information, he is believed to have been responsible for several attacks in Somalia using improvised explosive devices.