Officials said a motive for the attack Monday at the religious facility in the Brussels suburb of Anderlecht was not known, CNN reported Tuesday.
"One must remain extremely cautious when speculating on what happened," Anderlecht Mayor Gaetan Van Goidsenhoven told Belgian broadcaster RTBF. "The situation is extremely serious as someone has been killed."
The imam, 46, is thought to have died of smoke inhalation, police spokeswoman Marie Verbeke said.
The survivor was being treated for smoke inhalation, Verbeke said.
Van Goidsenhoven said people gathered outside of the mosque after the attack, but so far "things are quite calm."