A Somali official says Ethiopian troops arrested a former extremist leader who is now a candidate in a regional election in the southwest.

Nur Ahmed, an electoral official in Somalia’s Southwest state, told The Associated Press that Sheikh Mukhtar Robow, previously the number two leader of the rebel group al-Shabab, was arrested Thursday by Ethiopian troops that are part of the African Union forces supporting the Somali government.

The reason for the arrest is unclear.

Robow defected from al-Shabab last year and is now running to be regional president of Southwest state.

According to witnesses, the Ethiopian soldiers arrested Robow at the regional president’s residence in Baidoa. Officials said Somali police accompanied Ethiopian soldiers in the arrest.

Gunfire was heard in Baidoa as some of Robow’s supporters protested his arrest.