Pakistan is fighting a bloody campaign against al-Qaeda and Taliban extremists based near the border, who have launched dozens of suicide attacks over the past year, often using small cells in major cities.
Another senior Pakistani security official said the explosives used in the bombing were like those used in two other large-scale militant attacks, including one on the Danish embassy in Islamabad in June
''We are collecting evidence. The explosives were similar to those used in the Danish embassy [attack], which was claimed by al-Qaeda
, and the attack on the ISI [Pakistani security service] camp in Rawalpindi last year.''
Al-Qaeda leader Mustafa Abu al-Yazid, said the Danish embassy attack, which killed six people, was executed in revenge for Danish newspaper caricatures of the Prophet Muhammad.
No group claimed responsibility for a bus bombing near an ISI establishment in Rawalpindi last November that killed at least 15 people.
Afghanistan and Pakistan had agreed to ''honest cooperation'' against terrorism after the Marriott bombing, Afghan President Hamid Karzai's office said yesterday
Mr Karzai, whose country has been hit by similar attacks, called Pakistani Prime Minister Yousuf Raza Gilani on Sunday to offer his nation's condolences.
Mr Karzai told Mr Gilani the attack showed the only way to get rid of terrorism was widespread and honest cooperation in warring against it.
Mr Karzai's statement quoted Mr Gilani as saying, ''People of both countries are affected by terrorist attacks and a joint cooperation between both countries is needed