The suspects in the Boston Marathon bombing had originally planned to strike on July 4, but chose the race because it coincided with the time they had finished assembling the explosives, a law enforcement official said Thursday.
According to hospital interviews with surviving suspect Dzhokhar Tsarnaev shortly after he was captured, the suspects apparently finished constructing the explosives well before they had originally planned and chose to act sooner rather than wait, said the official who is not authorized to comment publicly.
It was not immediately clear, however, whether the suspects had identified a specific July 4 target that corresponded with a later completion time, the official said. But the suspects allegedly settled on the marathon after noticing preparations for the race shortly before the event.
Boston hosts one of the premiere July 4 celebrations in the U.S., featuring the Boston Pops and a spectacular fireworks display on the banks of the Charles River.
Tsarnaev has been charged with detonating one of the pressure-cooker devices, while brother Tamerlan Tsarnaev was killed in a confrontation with police in the days after the attacks.
Meanwhile, investigators in the Boston bombing case want to find out what Tamerlan Tsarnaev and his wife discussed when he phoned her a few hours after the FBI released photos of him and his brother as suspects in the deadly attack, a separate law enforcement official said Thursday.
Tamerlan Tsarnaev, 26, died not long after the conversation during a shootout with police that left his 19-year-old brother, Dzhokhar, seriously injured.
Dzhokhar Tsarnaev was captured a few hours later while hiding in a boat in a backyard in Watertown, Mass. He is currently being held at a prison medical center.
That same evening, police say, three classmates of Dzhokhar Tsarnaev at the University of Massachusetts-Dartmouth allegedly concluded that he was one of the suspects, went to his dorm room and removed his backpack and laptop. A federal complaint charges that they took the items to try to keep Dzhokhar Tsarnaev from getting into trouble over the bombings.
The content of the phone conversation between Tamerlan Tsarnaev and his wife, Katherine Russell, has not been disclosed, but authorities want to discuss it with her, the law enforcement official said.
The official said investigators are seeking Russell's cooperation in learning more about Tsarnaev's activities and communications leading up to the attacks.
Russell, a physician's daughter, met Tamerlan at Suffolk University, according to her lawyer. She converted to Islam and married him in 2010.
Russell's attorney, Amato DeLuca, said in a written statement Wednesday that his client "will continue to meet with law enforcement, as she has done for many hours over the past week, and provide as much assistance to the investigation as she can."
Earlier this week, federal investigators took a DNA sample from Russell as part of the inquiry that also involves analysis of remains of a pressure-cooker device detonated at the Boston Marathon finish line.
Authorities believe the pieces contain DNA and fingerprints that could assist in tracking the origin of the components or possible accomplices.
On Monday, FBI agents also visited the North Kingstown, R.I., home of Russell's parents, where she has been staying, and carried away several bags, according to FBI spokesman Jason Pack. Russell did not speak to reporters as she left her attorneys' office in Providence later in the day.
Attorneys have previously said Russell and her family were in shock when they learned of the allegations against her husband and brother-in-law.
In his written statement, DeLuca also noted that his client had been informed that the Massachusetts Medical Examiner's Office was prepared to release Tamerlan Tsarnaev's remains, but that she wanted them to be released to the Tsarnaev family.
"Katherine and her family continue to be deeply saddened by the harm that has been caused," DeLuca said in the statement. "They mourn for the loss of life and the terrible consequences these events have had for those who have been injured and for their families."
