About 150 people were evacuated from homes around the mosque in Walsall, West Midlands, after bomb disposal experts were called to the scene.
The item, which has not yet been identified, was found on Friday evening but police were not called until it was taken inside by a member of the public on Saturday.
Polce said they are “keeping options open” and added that no threats were made to people using the mosque.
Supt Keith Fraser, from the West Midlands force, said detectives were pursuing a number of lines of inquiry and continuing to work hard to establish what the item is and how it got there.
A number of mosques have been targeted since the killing of soldier Lee Rigby in London last month.