A videotape and letter have been handed to the BBC in which Pakistan's banned Sunni militant organisation Lashkar-e-Jhangvi claims it killed 50 Shia Muslims in a mosque in the city of Quetta 11 days ago.
By Paul Anderson
BBC correspondent, Islamabad
The letter said a series of attacks against Shia Muslims sent a message of protest against the Pakistan's Shia minority and the government.
The army took control of the city to stop revenge killings
Pakistani Interior Minister Faisal Saleh Hayat has said the investigation is advancing into the killings.
The videotape handed to BBC reporters in Quetta showed three men who claimed to belong to Lashkar-e-Jhangvi.
Only one talks to a still camera - the youngest, who is described as a 17-year-old suicide attacker.
He delivers a 10-minute anti-Shia diatribe.
He and his colleagues' appearance match those of three men who were identified by the police as the culprits in the mosque killing spree.
Pictures of their bloodied faces were published in newspapers after the attack.
The author of an accompanying letter, dated after the mosque attack, claims responsibility on behalf of Lashkar-e-Jhangvi for it and a string of attacks, including one the previous month in which 12 Shia police cadets were killed in the back of their truck by drive-by attackers.
The letter says all the attacks were a protest against the government, President Pervez Musharraf, the United States and Iran.
The letter says the mosque attackers sacrificed their lives to protect the honour of the Prophet Mohammed and his wife.
The dispute over the heirs to Mohammed is what lies at the hear of the dispute between Sunnis and Shias 1,400 years ago.
The Iranian revolution, led by Shia clerics, exacerbated the enmity.
It is not clear how much the admission of responsibility will affect the investigation.
To date dozens of people have been detained. Some have been released.
But between 30 and 35 suspects remain in custody under arrest.