As the first funerals and a giant memorial service brought 70,000 mourners and well-wishers to Littleton, the suburban community outside Denver that had seemed such a paragon of middle-class contentedness, police and judicial authorities wavered drastically in their public statements.
Investigators had grown increasingly sure that Eric Harris and Dylan Klebold, the two gunmen who died in the attack, must have had help, if only to transport more than 30 explosive devices, including a large propane bomb intended to blow up the cafeteria.
On Saturday, however, Jefferson County Sheriff John Stone said they had a found a diary belonging to one of the two gunmen indicating that they had been planning the attack for up to a year. The diary detailed good places to hide, times of day when the cafeteria was at its fullest, hand signals the gunmen should use, and a precise countdown to "rock and roll time".
The diary did not appear to mention accomplices, and yesterday officials shifted their line from "when" they find further suspects to "if".
On Friday, Jefferson County spokesman Jim Parr said the investigation was putting together a profile of a third gunman, based on eyewitness testimony. But statements over the weekend suggested this assumed that a gunman in a white T-shirt was someone other than Harris or Klebold.
At least one student has testified that Harris was wearing a white T- shirt under his trench coat, and officials are now not sure if there was a third gunman at all. "Maybe we're back to two suspects," a Sheriff's spokesman, Steve Davis, said.
Mr Davis had given considerable credence to a "death manifesto" supposedly written by Harris the day before the shootings in which he blamed teachers and parents for driving him to violence and threatening further bloodshed by 26 April. But within hours, he backtracked, saying the message appeared to have been written after the shootings and could not have originated with the killers.
Funerals are scheduled for much of the coming week. The bodies of the two killers have not yet been released by the coroner's office, but Klebold's family held a quiet private service for him on Saturday. In a statement, they said they loved him "as much as we knew how to love a child". Their grief over the tragedy was "indescribable".
The investigation is the biggest and most complex in Colorado's history, involving around 150 agents from local law enforcement bodies, the FBI and the Bureau of Alcohol, Tobacco and Firearms. More than 2,000 pieces of evidence have been collected from the school, and hundreds of interviews conducted.
No arrests have been made, meaning that any other suspects will have had ample time to destroy evidence. Three or four young people were taken into custody in the immediate aftermath of the attack, but it seems they were picked up simply because they were wearing leather jackets. They were released shortly afterwards.
The most likely arrests at this point appear to be the parents of one of the dead gunmen who apparently failed to notice explosive-making equipment in the house and the sawn-off part of a shotgun in their son's bedroom. Law enforcement officers have indicated such lapses might be grounds for charges of criminal negligence.