The Federal Bureau of Investigation (FBI) is offering up a $5million in reward for information that will lead to the arrest of an American citizen on their Most Wanted Terrorist List.

In an audio transcript of “Wanted by the FBI,” Supervisory Special Agent Erin Westfall said that Jehad Serwan Mostafa is believed to be the highest-ranking U.S. citizen fighting overseas with a terrorist organization, and the alleged leader of al-Shabaab.

“For about a decade, Mostafa has held positions within in al Shabaab's explosives department, media wing, and training camps,” she said.

FBI poster A U.S. Federal Bureau of Investigation poster shows photographs of seven men identified as Russian intelligence officers who were indicted in Washington, D.C. Oct. 4, 2018. Photo: Federal Bureau of Investigation/Handout via REUTERS

Westfall added that unless he is stopped, the agency is keen on their belief that Mostafa will continue to be involved in al Shabaab's terrorist acts.

Meanwhile, U.S. Attorney for the Southern District of California Robert Brewer said in a news release that Mostafa is also linked to the use of Improvised Explosive Devices (IEDs) in attacks in Somalia, while the insurgents' motives to overthrow the Somali government and promote an Islamic state is a matter not to be taken lightly.

“Al-Shabaab's reign of terror threatens U.S. national security, our international allies and innocent civilians,” he said.

An indictment that was unsealed in federal court Monday accused the 37-year-old Mostafa of “providing material support” to the terrorist acts of al-Shabaab, according to CNN.

San Diego special-agent-in-charge Scott Brunner said Mostafa is known to use a number of aliases, including “Ahmed Gurey,” “Anwar,” “Ahmed,” “Abu Abdallah al Muhajir” and “Abu Anwar al Muhajir.”

Authorities pointed that Mostafa grew up in a Serra Mesa neighborhood in San Diego and attended high school and college there.

After leaving San Diego in 2005, Mostafa first traveled to Yemen and then to Somalia where he fought against Ethiopian forces and later joined al-Shabaab.

Al-Shabaab was declared a terrorist group in 2008. A year later, Mostafa was indicted on similar crimes but he has remained at large.