Al Jazeera's Kamal Hyder, reporting from Islamabad, said several clues had been found.
"Police sources have revealed to Al Jazeera that they have been able to get two other vehicles with the same license plate number. That would suggest that there was a big plan for other attacks," he said.
"They would not say whether they had busted the gang which was behind this, but security sources said they have obtained vital clues, as well as forensics, to be able to start their investigations."
'One-off attack'
An unnamed government official told AFP news agency on Tuesday that pro-Taliban fighters were the "most likely" perpetrators of the attack.
He also said that the attack would not harm peace talks between Pakistan's new government and armed groups operating in the tribal regions bordering Afghanistan.
"It appears to be a one-off attack which has little relevance to the ongoing negotiations between [the] Taliban and the authorities," he said.
"This attack was not born out of the events in the country or the region, rather it was part of global outrage in the Islamic world against publishing blasphemous cartoons."
Danish newspapers had republished cartoons satirising the Prophet Muhammad in February. It sparked anger across many Muslim majority countries.
However, Ahmed Qureshi, a political analyst, told Al Jazeera that the timing of the attack might indicate it was not a backlash against the cartoons.
"The timing is very strange," he said.
"It is not an issue right now, it's over. No one, not even in the tribal areas, is talking about the cartoons."
'Pointing fingers'
Ahmed Latif, a senior police officer, said the bombing was likely to have been a suicide attack.
"I think it is too early to really name someone ...
immediately after the attack takes place, people like to blame someone."
Khalid Rahman, director-general, Institute for Policy Studies |
No group has so far come forward to claim responsibility.
Khalid Rahman, the director-general of the Institute for Policy Studies, told Al Jazeera that it is too soon to start speculating on who may be responsible for the attack.
"I think it is too early to really name someone as the culprit, and perhaps this has been the tragedy. Immediately after the attack takes place, people like to blame someone," he said.
"But to blame someone, one needs to conduct a proper and deeper investigation.
"Since 2001, we have had more than 60 such incidents, but unfortunately, fingers are pointed as to who may be responsible, but genuine information is never provided."
'Poor execution'
Security officials said that the explosives were of a type used in previous attacks attributed to pro-Taliban fighters in the tribal areas bordering Afghanistan.
The preparations for the blast were "meticulous, similar to previous attacks by Taliban linked to al-Qaeda" and involved a car stolen from the northwestern city of Peshawar, the official said.
However, the attack itself was "poorly executed" and the bomb went off several metres from the gate of the embassy. Several diplomatic buildings and neighbouring houses were damaged.
Kamal Shah, the interior secretary, said that evidence showed the car was a Toyota Corolla carrying 25kg of explosives.
According to another security official, the same type of explosive was used in a suicide blast at the offices of the Federal Intelligence Agency (FIA), in the eastern city of Lahore in March.