Questions are still being raised about the unsolved murder of three people in Boston more than a year before the bombings.
It was a shocking slaying in a Boston suburb that sometimes goes years without one homicide, let alone three at once. The victims' throats had been sliced in a home on a tree-lined street, marijuana and cash strewn over their bodies.
The investigation languished for more than 18 months until just a few days after the Boston Marathon explosions, when the FBI identified the suspected bombers as Tamerlan Tsarnaev, 26, and his brother, Dzhokhar Tsarnaev, 19, ethnic Chechens who emigrated from Russia to the US about a decade ago.
The 2011 homicide case in Waltham remains unsolved, though Tamerlan Tsarnaev was fingered in the killings after his death in a post-bombing shootout with police. Clues in the Waltham killing that might early on have led local investigators to Tamerlan Tsarnaev, who was already on the FBI's radar as a possible religious extremist, raise questions about whether the marathon bombings could have been averted.
"Let's put it this way: If they had arrested Tamerlan as one of the killers in the triple killing before the marathon bombing, it certainly would have affected the outcome of the marathon bombing," said Waltham City Councillor Gary Marchese.
"He would have been in jail and would have been tried for murder."
Tamerlan Tsarnaev and one of the slaying victims, Brendan Mess, were both boxers and sparred often at a local gym.
Despite what friends described as a close friendship, Tamerlan did not attend Mess' funeral. And the owner of a restaurant where Mess was a regular and brought Tamerlan and another victim along with him a few times says police never interviewed him.
The brothers lived in Cambridge, across the Charles River from Boston.
In March 2011, months before the September killings to the west in Waltham, Russian authorities had told the FBI they worried that Tamerlan Tsarnaev and his mother, who lives in Russia, were religious extremists.
The Russians were unresponsive when pressed for more details. The FBI didn't find any derogatory information on Tsarnaev, and a criminal case was not opened. The FBI shared its results with Russia in summer 2011, shortly before the Waltham killings.
The US government has wrestled with what more it could have done to prevent the marathon bombings, which killed three people and injured more than 260 others a year ago on Tuesday (local time). The FBI believes the Tsarnaev brothers acted alone and had no ties to foreign terrorist organizations.
A yearlong federal review into strands of information intelligence agencies had before the explosions found investigators could have asked more questions and shared more. But the review concluded it was unclear whether any additional steps by federal agencies could have prevented the attack.
Within weeks of the marathon bombings, Massachusetts authorities went to Florida to interview a friend of Tamerlan's, Ibragim Todashev, who implicated both himself and Tamerlan in the killings in the home in Waltham, a middle-class city of more than 60,000.
That it took more than 18 months for law enforcement to get a lead on the 2011 slayings has angered the victims' friends and relatives. They're asking the same pressing question the federal review didn't address: What might have happened on April 15, 2013, if one of the bombing suspects had been arrested months earlier for the Waltham deaths?
"If investigators didn't write it off as just another three drug dealers dead and assumed that these deaths were not as important, then maybe the Boston Marathon bombing might not have happened," said Krysta Voskowsky, one of Mess' friends.
Law enforcement isn't talking. County prosecutor Marian Ryan and Waltham Police Chief Keith MacPherson, who inherited the case after taking office, have called the investigation active and will not answer questions about whether Tamerlan Tsarnaev is a suspect.
Waltham detectives referred questions to Ryan's office. Gerry Leone, the county prosecutor at the time of the killings, has declined to comment.
"It's been an amazing black hole of information," said Marchese, the city councillor. "There's nothing."
No one has indicated that Tamerlan's brother, heading for trial in the bombings, is also a suspect.
The US has long been worried about a domestic attack carried out by ideologues not tied to a larger terrorist group.
A mantra emerged after the September 11 attacks - "Homeland security is hometown security" - meaning local police are the first lines of defense in rooting out terrorists.
In Waltham, property crime is the most routinely reported offense, according to FBI reports.
Murders are so rare that a police map detailing reported crimes doesn't even list homicide as a search category. So when Mess, Erik Weissman and Raphael Teken were found nearly decapitated on September 12, 2011, it came as a shock.
Police initially believed the killings were drug-related. Mess was known as a pot dealer. Weissman, 31, had been arrested on drug charges. Teken, 37, was a personal trainer.
The killings stunned residents of the dead-end street where Mess lived.
Hours after the bodies were found, a prosecutor told reporters that there was no sign of a break-in and that the victims probably knew their killers.
Little else was heard about the investigation until after the marathon, but details are few on how police made the connection.
Tamerlan Tsarnaev, Todashev and Mess trained at the same Boston gym. Todashev was shot dead in May by an FBI agent during an interrogation at his home near Orlando. Authorities recently cleared the agent, saying Todashev had charged at a Massachusetts trooper.
Federal authorities have said that Todashev implicated Tamerlan in the Waltham slayings.
But at the request of Massachusetts prosecutors, the Justice Department did not release details of the confession.
It's unclear which leads investigators might have pursued immediately after the Waltham deaths.
Jamal Abu Rubieh, owner of Brookline Lunch, the restaurant in Cambridge that Mess frequented, said he was surprised he never heard from police. Weissman and Tamerlan Tsarnaev ate there with Mess a few times.
It's routine for investigators of a killing to talk to anyone who had regular contact with the victim.
"Nobody ever came to talk to us," Rubieh said.
Even though officials insist the state is running the investigation - standard procedure except in Massachusetts' largest cities - federal authorities also appear interested.
In January, a federal grand jury subpoenaed Tsarnaev's wife, Katherine Russell, asking her for things that belonged to Mess, including a leather jacket, said Amato DeLuca, her lawyer.
County and federal prosecutors declined to comment on the subpoena. It's not clear why the federal grand jury would want Mess' belongings, and Russell's lawyer said she does not have anything that belonged to Mess.