Shortly after the Saturday bombings in the heart of Delhi, police and investigating teams got down to work.
Forensic experts and detectives scoured the site of the attacks, looking for traces of the explosives used and any other leads that could suggest who carried out the attacks.
The bombs hit markets crammed with shoppers
Simultaneously, raids were launched on guesthouses in a number of areas to look for suspects.
Many people were taken in for questioning but there have been no arrests as yet.
But on Monday, India gave its clearest indication that it believes that Islamic militant groups based in Pakistan, or having links to them, were involved in the bombings.
In a telephone conversation with Pakistani President Pervez Musharraf, Prime Minister Manmohan Singh said India expected Pakistan to act against terrorism directed against it.
The investigating authorities believe that an attack of this magnitude and sophistication - with three bombs going off in near succession and with great intensity - could only be the work of a few groups known to them.
In the past, suspicion has always fallen on several Kashmiri militant groups such as the Lashkar-e-Toiba, one of the most powerful groups and based in Pakistan.
More recently, an attack in May on two cinemas in Delhi was blamed on Sikh militant groups.
India has taken its time before pointing the finger at Pakistan-based groups, and its public rhetoric after the attacks has been uncharacteristically measured.
It hasn't always been the case.
In December 2001, when gunmen attacked India's parliament building - an attack in which 14 people died - Delhi was quick to blame Pakistan for allegedly harbouring groups that were behind that attack.
In the months that followed, relations between the two countries nose-dived and they came close to an all-out war in the summer of 2002.
But this time, it is clear that they do not want their relations to deteriorate to that level.
Both countries have made major progress in their relations.
Pakistan was swift to condemn the Saturday attack, describing it as barbaric.
And hours after the blasts, both countries announced a landmark agreement on opening the de facto border in disputed Kashmir, to allow relief to reach victims of the recent earthquake.
If, as some have suggested, the motive behind the Delhi bombs was to derail the peace process, there is no indication yet that it will have that effect.
Never heard of
So who is behind this latest attack?
A group calling itself the Islami Inqilabi Mahaz (Islamic Revolutionary Group), claiming to be based in Kashmir, says it carried out the attack.
Police respond to the attack on parliament in Delhi, 2001
But Kashmir analysts say they have never heard of the group.
Delhi Police chief, KK Paul, told the BBC that they had some knowledge of the group.
"We had come to know about the group some five or six years ago.
"It is not altogether an unknown group."
But it is still unclear whether the group still exists and indeed, whether it had anything to do with the attacks.
Indian security analysts say that the finger of suspicion has to point to Kashmiri groups.
"There are only two groups capable of carrying out these types of attacks," says security analyst Brahma Chellaney, "the Lashkar-e-Toiba and the Jaish-e-Mohammad."
Both groups are based in Pakistan, although they have been outlawed, and have been listed as terrorist groups by the US State Department.
Saturday's attack also signalled a major departure from previous attacks.
It targeted innocent civilians - middle-class Indians out shopping ahead of the festival season - instead of symbols of power, such as politicians and government buildings as has been the case in the past.
Defence analyst and former Indian army major, Maroof Raza, says the attack may be a sign of things to come.
"It is clear that the militant groups are trying to make a comeback.
"It is also clear that they have shifted their focus, since it's becoming increasingly difficult to successfully launch an attack on high-profile targets," he told the BBC News website.
Major militant groups operating in India are usually quick to own up to attacks, especially those aimed at the security forces or government.
But they rarely acknowledge attacks on civilians.
It is possible that the attacks may never be credited to those who carried them out.
But equally, it does not appear at present that it will upset the delicate relations between India and Pakistan - despite the latest comments from Manmohan Singh.
The spokeswoman for the Pakistan foreign ministry, Tasnim Aslam, says Pakistan does not know who carried out the attack, but they would have to be defeated.
"There is a realisation on both sides that we have to defeat their aims and we have to rise above this and we have to continue this peace process for the sake of the people on both sides," she said.