New details are emerging on the suspects' mindset during the Boston bombing. Also this week, three friends of Dzhokhar Tsarnaev were arrested. They handed over Tsarnaev's laptop to the FBI and led investigators to discover a backpack belonging to the suspect which had been thrown away and then recovered. NBC's Pete Williams reports.

The brothers accused in the Boston Marathon bombings originally planned to set off explosives on July 4 — but changed their minds and decided on Patriot’s Day, officials with knowledge of suspect Dzhokhar Tsarnaev’s interrogation told NBC News.

Investigators also now have a good idea of how and where the pressure-cooker bombs used in the attack were constructed. But they still are trying to determine where the components in the explosives came from.

Officials say Tamerlan Tsarnaev, 26, and his brother Dzhokhar Tsarnaev, 19, dropped pressure-cooker bombs stashed in backpacks in crowds watching runners at the finish line of the marathon on April 15.

Dzhokhar allegedly told his interrogators the weekend after he was arrested that he and his brother originally intended to set off their bombs somewhere on the Fourth of July, officials said.

Investigators say they have recovered the accused Boston Marathon bomber's computer, as three of Tsarnaev's friends being held by police insist they will cooperate with the investigation. NBC's Jay Gray reports.

But, according to the officials, Dzhokhar said that when the brothers finished the bombs sooner than they expected, they decided to stage the attack earlier, on Patriot's Day, a proud holiday in Boston that hosts the marathon, a Red Sox game and other festivities.

Officials say the surviving suspect also said the bombs were built in the house of his brother, Tamerlan. Authorities had previously disclosed that explosive residue had been found there.

And a backpack that officials say was snatched from Dzhokhar Tsarnaev’s dorm room then thrown away by three of his friends before being recovered is providing some clues on the bombs.

Federal agents recovered the backpack in the New Bedford, Mass., landfill. It contained fireworks tubes that had been emptied of their powder.

According to the FBI, the found fireworks were from a store other than the Phantom Fireworks shop in Seabrook, N.H., which disclosed a purchase by Tamerlan Tsarnaev in February.

The FBI was checking on sales with other dealers in the area.

In addition, the FBI said the backpack contained a jar of Vaseline petroleum jelly.

"Bombers use Vaseline as a binder and thickening agent,” an FBI investigator said. “They mix it with explosive powder in hopes of making the mixture a little more reliable."

The use of Vaseline is not mentioned in the al Qaeda online magazine that investigators believe the brothers used as a blueprint to make their bombs, but it is widely discussed elsewhere on the Internet.

Authorities say the three friends — who have been hit with obstruction of justice and lying charges — also took Dzhokhar Tsarnaev's laptop computer from his dorm room but did not throw it away and have since given it to the FBI.

Meanwhile the body of Tamerlan Tsarnaev was moved to a funeral home on Thursday evening at the request of family members, according to the Massachusetts Medical Examiner’s Office.

Soon after receiving the body, the funeral home must file a death certificate, which would make public Tsarnaev’s cause of death.

Tamerlan Tsarnaev was killed in a firefight with police in the early morning hours of April 19 in the Boston suburb of Watertown. His precise cause of death, however, has remained unknown, since a death certificate or autopsy results have not been made public.

The name of the family members who requested the release of the body and the funeral home were not released.

Dzhokhar Tsarnaev is in a prison hospital awaiting trial on terrorism charges.

He has told investigators the brothers acted alone in concocting the deadly plot, and planted the bombs to defend Islam after the U.S.-led wars in Iraq and Afghanistan, law enforcement officials told NBC News.

Related:

Who's who in the Boston Marathon bombing investigation

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