The FBI has offered a $5 million reward to help catch an American citizen on the agency’s most wanted list - amid fears he could be plotting an attack in the US.

Officials unsealed a new indictment against 37-year-old Jehad Serwan Mostafa, claiming his leadership of the terrorist outfit al-Shabaab has led to the slaughter and injury of hundreds of innocent victims in Africa.

Regarded now as the highest-ranking US citizen fighting overseas with a terrorist organisation, Mostafa was first charged in 2009 with several counts of providing material support to a foreign terrorist organisation – however he has remained at large.

But now, under the new superseding indictment, Wisconsin-born Mostafa has been accused of playing a critical role in al-Shabab, which promotes an Islamic state and has killed thousands of people since its inception in 2006.

Officials unsealed a new indictment against 37-year-old Jehad Serwan Mostafa on Monday, claiming his leadership of the terrorist outfit al-Shabaab has led to the slaughter and injury of hundreds of innocent victims in Africa

Under the new superseding indictment, Wisconsin-born Mostafa has accused of playing a critical role in al-Shabab, which promotes an Islamic state and has killed thousands of people since its inception in 2006

The charges announced Monday accuse Mostafa of helping to train soldiers, planning attacks on the Somali government and African Union forces, helping lead the group’s explosives department, and overseeing the group’s media operations.

Investigators also say Mostafa, who they describe as ‘extremely dangerous’, has played an influential role in various other acts of violence in Africa, spanning between 2009 and 2017.

Investigators also say Mostafa, who they describe as ‘extremely dangerous’, has played an influential role in various others acts of violence in Africa too, spanning between 2009 and 2017

However, FBI leaders fear Mostafa’s US roots could be used to plot an attack either in America or against Americans abroad, following a recent call inside al-Shabaab for attacks to be carried out on US citizens.

‘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,’ US Attorney for the Southern District of California Robert Brewer said.

FBI special agent Scott Brunner echoed: ‘We must use every tool in our toolbox to disrupt a potential terrorist attack before it happens.’

Mostafa has remained a key figure on the FBI’s Most Wanted Terrorist List for nearly a decade. The Bureau is now asking citizens around the world to study his picture and listen to his voice to help finally bring him to justice.

‘The FBI, the US Attorney’s Office, and the State Department have continued an extensive investigation since 2010 to locate this terrorist,’ explained Brunner. ‘The FBI has not forgotten, has not stopped working, and will not stop until Mostafa is found.’

Mostafa has remained a key figure on the FBI’s Most Wanted Terrorist List for nearly a decade. The Bureau is now asking citizens around the world to study his picture and listen to his voice to help finally bring him to justice

Born in Waukesha, Wisconsin, Mostafa spent his formative years in San Diego, where he went on to attend the University of California.

Graduating in 2004, the following year he travelled to Yemen and Somalia, where he fought against Ethiopian forces. He then joined al-Shabaab in 2006, investigators believe, where he quickly began rising up the terror group’s ranks.

At the time of his original indictment, friends of Mostafa growing up in Serra Mesa described the now infamous terrorist leader as a ‘quiet’ and ‘polite’ person who often promoted Islam as a religion of peace and love.

One woman who asked not to be identified, told NBCSanDiego said she knew Mostafa from the time he worked at the front desk of an auto shop on El Cajon Blvd.

‘He was the exact opposite of what you'd think of a terrorist. Very kind, very peaceful, very patient. Not scary or aggressive,’ she said. ‘When people talked that Muslims are bad and Muslims are scary, I’d say I’ve got a Muslim mechanic and he’s so sweet.

'He’s what I used as an example of a good Muslim. To hear he’s in a terrorist group, let alone the one he’s in, is bizarre.’

Mostafa’s father, Abdul Halim Mostafa, died last year but was said to be a well-respected man among the local community. Calls to the family home went unanswered when contacted by DailyMail.com.

Shortly before Mostafa departed the US, he married a Somalian woman.

In 2005 Mostafa ravelled to Yemen and Somalia, where he fought against Ethiopian forces. He then joined al-Shabaab in 2006, investigators believe, where he quickly began rising up the terror group’s ranks.

Officials say they believe Mostafa - who is also known as ‘Ahmed Grey, ‘Ahmed’, ‘Anwar’, ‘Abu Anwar al Muhajir’ and ‘Abu Abdallah al Muhajir’ - is in Somalia currently.

Federal prosecutors in San Diego say al-Shabaab’s attacks have led to the injury and deaths of hundreds of African people.

In 2010, al-Shabaab carried out a suicide bombing at a bar in Uganda during a World Cup soccer match, killing 76 people including a US citizen who had been working in the region for a San Diego-based non-profit.

A 2013 attack carried out by the terrorist outfit at a shopping Mall in Nairobi, Kenya, killed dozens and left more than 150 injured when attackers dressed in black threw grenades at the unsuspecting crowds and sprayed them with bullets from AK-47s. Among the dead was a graduate of a San Diego-area high school.

Additionally, on January 15, 2019, al-Shabaab executed a coordinated attack at the DusitD2 hotel, in Nairobi, where 16 innocent civilians were killed, including one U.S. citizen.

If you have any information concerning this person, please contact your local FBI office or the nearest American Embassy or Consulate. You may also call 1-800-CALL-FBI (1-800-225-5324), or submit tips online via tips.fbi.gov.