Four men are being held in Pakistan in connection with the 19 March bombing of a Shia Muslim shrine that killed 43 people, police have revealed.
Worshippers were killed as they sat down to eat
The suspects were detained shortly after the blast at a shrine near Naseerabad in Balochistan province, police chief Chaudhry Yaqub said.
He said the men were all members of the Sunni militant group, Sipah-e-Sahaba, which was banned in 2002.
"They are terrorists and their trial will be held very soon," he said.
Mr Yaqub told a news conference in the Balochistan capital, Quetta, that the men had "confessed to masterminding and carrying out the bomb attack".
He did not say why police were only now revealing details of the suspects' arrests. None of the suspects or their lawyers were available for comment.
Thousands of people had gathered in the village of Fatehpur, about 300km (185 miles) from Quetta, when the powerful bomb exploded.
A second bomb was found at the shrine and safely removed.
Casualty figures at the time were put at 30 - Mr Yaqub said a number of people had since succumbed to their injuries, taking the toll to 43.
Balochistan has a history of violence between Shia and Sunni Muslims and has also seen increased attacks by tribesmen fighting for more autonomy.
Correspondents said the attack did not fit into Pakistan's pattern of sectarian violence, as both Sunnis and Shias were among the casualties.
The tribesmen's targets have usually been security forces and the power and rail infrastructure, not large gatherings of civilians.