A Glendale Police Department spokesman said two men driving in a red car threw a soda bottle filled with acid and a reactant at the Albanian American Islamic Center of Arizona, in Glendale, west of Phoenix.
The bottle, which contained pool cleaner and strips of tin foil, burst some 20-25 feet away from Imam Didmar Faja and another mosque official, although neither man was injured, sergeant Jim Toomey said.
"The bottle ruptured in front of them and they smelled a strong chemical smell when it went off," Toomey said.
"We are treating it as a hate crime. We are taking it very seriously," he added.
Faja is one of six Muslim clerics known as the "Flying Imams" who are bringing a suit against US Airways alleging discrimination after they were removed from a Minneapolis to Phoenix flight last November.
The imams were ordered off the aircraft and were briefly detained by authorities after some passengers and crew became alarmed at what they believed was suspicious behavior.
The Council on American-Islamic Relations said on Wednesday that the imams and their attorney had received death threats stemming from the lawsuit, and urged Glendale police to consider that as a possible line of inquiry in their investigation.
Toomey said that the attack was one of six similar incidents involving the soda-bottle device in the Glendale area over a three-day period, but the other five did not appear to have a religious link.
"Until we know (the reason), we are going to assume that (the mosque attack) was religiously motivated," he added.
In the days after the September 11, 2001, attacks, the Phoenix valley area was the scene of a widely reported hate crime in which a Phoenix man shot dead a Sikh outside his gasoline station.
The victim, Balbir Singh Sodhi, was one of several Sikhs attacked in the United States after apparently being mistaken as possible supporters of Saudi-born Osama bin Laden, believed to be behind the attacks.