They said they had collected evidence inside the building and would make more details available on Tuesday. They declined to comment on the potential motive.
Two of the three men -- aged 30, 35 and 56 -- were seriously injured in the attack shortly after 5:30 p.m. local time (1630 GMT) near the main train station in Switzerland's financial capital, Zurich police said.
A third sustained less severe injuries. All three were brought to hospital.
The unidentified suspect, a man around 30 years old who according to witnesses was wearing dark clothing and a dark wool cap, fled the mosque, police said.
Police said a body was found nearby but would not comment on any link to the shootings while investigations continued.
People at the scene told Reuters the Islamic Center on Zurich's Eisgasse was used as a mosque, often by Somalis.
"We never once had a problem," said Abukar Abshirow, a Somali who said he was a regular worshipper at the center that attracted Muslims from around the world.
"We never had anyone come and say why are you here. We never had that," Abshirow said. He said the three victims were Somalis.
Two thirds of Switzerland's 8.3 million residents identify as Christian but the nation has been wrestling with the role of Islam as its Muslim population has risen to 5 percent, swelled by the arrival of immigrants from the former Yugoslavia.
In 2009, a nationwide vote backed a constitutional ban on new minarets.
The Federation of Islamic Organisations in Switzerland said the center was not a member and it did not have any direct knowledge of the incident.
Banner Image Credit: Reuters