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The suspects in the Boston Marathon double bombing had originally planned to strike on July 4, but chose the race because it coincided with the time they had finished assembling their explosives, law enforcement officials 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 one of the officials, who was not authorized to speak publicly on the matter.

It was not immediately clear, however, whether the suspects had identified a specific July 4 target, the official said. 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.

The suspects allegedly settled on the marathon after noticing preparations for the race shortly before the event.

Tsarnaev has been charged with detonating one of the pressure-cooker devices; his brother Tamerlan Tsarnaev was killed in a confrontation with police in the days after the attacks.

Meanwhile, investigators want to find out what Tamerlan Tsarnaev and his wife, Katherine Russell, discussed when he phoned her a few hours after the FBI released photos of him and his brother as suspects in the deadly attack.

Tamerlan Tsarnaev, 26, died not long after the April 18 conversation during a shootout with police that left Dzhokhar, 19, seriously injured.

Dzhokhar Tsarnaev was captured a few hours later hiding in a boat in a backyard in Watertown, Mass. He is being held at a prison medical center.

A spokesman for Russell's attorney, Amato DeLuca, did not respond directly to questions about Russell's telephone contact with Tsarnaev. Instead, he referred to a written statement DeLuca issued earlier that said Russell "will continue to meet with law enforcement, as she has done for many hours over the past week, and provide as much assistance ... as she can."

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 race finish line.

On the same evening that Tamerlan and Russell spoke by telephone, federal prosecutors now allege that three classmates of Dzhokhar Tsarnaev at the University of Massachusetts-Dartmouth concluded that he was one of the suspects after seeing the photographs circulated by the FBI. The three, charged Wednesday with impeding the investigation, allegedly went to his dorm room and removed his backpack containing fireworks stripped of gunpowder and a laptop. Gunpowder was among the main components in the bombs.

A federal complaint charges that they took the items to try to keep their friend from getting into trouble. The backpack was recovered at a Boston-area landfill. The laptop has since been turned over to authorities.

Authorities do not believe the students knew about the attack plan or helped carry it out.