A letter recovered from the pocket of one of the attackers of a Shia mosque in Karachi on Monday warns of more terror attacks, police say.
Sectarian violence in Karachi has claimed 100 lives in the past year
The letter was signed by the current operational head of a Sunni militant outfit, Lashkar-e-Jhangvi, the Associated Press news agency said.
The letter said the group carried out the mosque raid, in which five died.
More than 100 people have died in violence between Sunni and Shia Muslims in Karachi over the past year.
Chief investigator Manzoor Moghul said the letter was recovered from one of the assailants who was shot dead by police.
It was signed by Asif Chotu, widely believed to be leading the Lashkar-e-Jhangvi after most of its top leadership were arrested or killed over the past two years.
"Yes, we have found a letter and this is Asif Chotu's style to leave a signature for an attack," Mr Moghul said.
Asif Chotu probably dropped the bombers off at the mosque, he said.
Police are interrogating one suspect - identified as Tehseen - who survived the mosque attack.
Five people, including two attackers, were killed and several others wounded.
Angry protesters set fire to a number of shops, gas stations and vehicles in the aftermath of the bombing.
Six employees of the American fast food chain KFC were burned to death when an outlet close to the mosque was torched by rioters.
Asif Chotu, aka Rizwan Ali, is also the prime suspect in last year's bombing of a Shia mosque that left 15 people dead and scores injured.
Investigators believe he started out as a rank-and-file gunman in 1992 but soon graduated to a senior position within Lashkar.
He is believed to be an expert in making bombs and is said to have trained Lashkar activists in suicide bombings.
Lashkar-e-Jhangvi was banned in 2001 for its role in fanning sectarian violence.