Government officials now say it was an act of sabotage — and that Britain and the United States provided support to the group that carried it out.
Both countries have denied similar allegations in the past.
Soon after the April 12 blast in the southern city of Shiraz, officials blamed it on a home-made bomb. They changed the account the following day and said it resulted from negligent handling of live munitions inside the mosque.
Officials launched an investigation.
On April 30, Minister of Information Gholam Hossein Mohseni Ejeie said "no reason, sign or witness has been found proving the possibility of sabotage," according to a report in the official Iranian news agency, IRNA.
However, the same minister claimed the blast was sabotage Wednesday — and said the suspects had links with certain Western countries, including Britain and the United States.
"Not only they did not not take any measure against them but also they rather supported them," the minister is quoted as saying in a report by the state news agency.
The explosion took place shortly after evening prayers.
Fars, another news agency, said about 800 people, mostly young, were gathered at the mosque to hear a prominent local cleric's sermon denouncing Bahai and Wahabi faiths — both of which are considered heresies by some Shiites, who make up about 90 percent of the Iranian population.
Fars' initial reports said a home-made bomb was to blame. Then officials said the munitions were apparently left behind after a "Sacred Defense" exhibition held at the mosque, which also serves as a cultural center.
"Sacred Defense" is a local reference to the Iran-Iraq war of the 1980s.
Local militia groups — known as Basij — often use the mosques for meeting places.
Bombings are unusual in Iran, though the predominantly Shiite Muslim country has endured sporadic attacks in recent years.
The last major bombing occurred in February 2007, when a car bomb blew up near a bus carrying members of Iran's Islamic Revolutionary Corps, leaving a dozen dead and injuring dozens more in southeastern Iran.
Shiraz — a historical city of more than 1 million people — is well known for being home to many scholars, artists, poets and local craftsmanship of rugs and metalwork. The tourist city, about 400 miles south of Tehran, the capital of southern province of Fars.
