Pakistani investigators said they believed a hardline Sunni militant group ordered the bloody suicide bombing of a Shiite mosque in Karachi this week, triggering a third consecutive day of rioting.
They suspect Lashkar-e-Jhangvi of being behind Monday's blast at the Ali Raza mosque, the death toll from which rose to 21 after officials confirmed another victim had died late Wednesday.
Lashkar-e-Jhangvi, banned in August 2002, was blamed for a similar suicide attack on another Shiite place of worship, the Haderi mosque, on May 7 in which 23 worshippers were killed.
"Lashkar was involved in the May 7 attack, which gives us ground to believe that the same terrorist outfit is involved in the recent attack," senior investigator Gul Hameed Sammo told AFP.
"We see the impact on the Ali Raza mosque building was similar to the Hyderi mosque and this bomb was even heavier."
Ten kilogrammes (22 pounds) of explosives were used in the Ali Raza mosque attack.
"This attack left the body of the suicide bomber torn into much smaller pieces than the one who blew himself up in the previous attack," Sammo said.
Lashkar-e-Jhangvi is one of the fiercest Sunni sectarian gangs in Pakistan and has been killing followers of the rival Shiite sect for more than a decade. Investigators suspect the Ali Raza mosque was attacked to avenge the murder the day before of high-profile Sunni cleric Mufti Nizamuddin Shamzai in the same neighbourhood.
"LJ (Lashkar-e-Jhangvi) is notorious for such revenge killings and we see the strong possibility of an act of revenge of Shamzai's killing," an investigator who asked not to be named said.
Shamzai's assassination may itself have been in revenge for the May 7 Shiite mosque attack.
Each attack and the funerals that followed triggered massive rioting and ransacking and torching of shops and cars. At least five people were killed in clashes between police and protestors.
The death toll from a month of violence in the commercial hub of Karachi now stands at 52.