The bomb detonated during evening prayers in the mosque
A provincial official in Iran has accused the United States of being behind Thursday's bombing of a mosque that killed at least 19 people.
Jalal Sayah, deputy governor of Sistan-Baluchestan province, said three people had been arrested following the attack.
"According to the information we obtained they were hired by America and the agents of arrogance," he said.
Some 60 people were hurt in the attack during evening prayers at the mosque in Sistan-Baluchestan's capital, Zahedan.
The city is mainly Sunni Muslim and the remote province is one of the most deprived in this mostly Shia country.
It comes at a time of heightened political sensitivity nationally, with just over two weeks before the
first round of the presidential election.
Zahedan's Amir al-Mohini mosque - an important Shia mosque in the city - was crowded with worshippers when the attack happened.
They had gathered for prayers on what was a public holiday to mark the death of the Prophet Muhammad's daughter, Fatima.
Iranian media said a suicide bomber had carried out the attack, and officials said after the blast that arrests had been made.
Sistan-Baluchestan borders both Afghanistan and Pakistan, making it a key route in the drugs trade.
Despite Iran's best efforts, a huge proportion of the world's opiates, such as heroin and morphine, are smuggled by heavily armed drugs gangs, often in large convoys.
There are also a number of militants in the area, many of them with links to the drugs gangs, and clashes with the security forces are common.
However, Jalal Sayah, in comments to the semi-official Fars news agency, accused the attackers of being mercenaries hired by the US.
It is a common accusation from the Iranians, the BBC's Jon Leyne in Tehran says, and the facts of this case may never be known.
But it is an open secret that former US President George W Bush directed large amounts of money to try to destabilise Iran and there is no sign the policy is any different under President Barack Obama, our correspondent adds.