Katherine Russell Tsarnaeva, wife of killed Boston Marathon bombing suspect Tamerlan Tsarnaev, outside her home.

Katherine Russell Tsarnaeva, wife of killed Boston Marathon bombing suspect Tamerlan Tsarnaev, outside her home.

One of the men believed responsible for the catastrophic Boston Marathon bombings last week bought the "most powerful" fireworks he could find two months before carrying out the attack.

It comes as police reportedly expect the victim of Dzhokhar and Tamerlan Tsarnaev's car-jacking last Friday to be a star witness in their case against the younger brother, preventing the case being thrown out because of inadmissible admissions made while Dzhokhar lies in hospital.

According to police sources quoted in US media, Dzhokhar Tsarnaev, 19, has said that Tamerlan, 26 – shot dead during an intense gun battle with police – was the driving force behind the blasts, which killed three people and injured hundreds more.

Overnight, Tamerlan's wife, Katherine Russell released a statement through her lawyer, saying the reports of her husband's involvement in the bombings caused her and her family "profound distress and sorrow".

In an interview with The New York Times, a senior employee of a chain of fireworks shops said Tamerlan visited one of his shops in New Hampshire on February 6.

"He came in and he asked the question that 90 per cent of males ask when they walk into a fireworks store: 'What's the most powerful thing you've got?' " vice-president William Weimer said.

Mr Weimer said that Tamerlan bought a reloadable mortar kit, but added that even if all the powder from the shells bought that day was taken, he did not think there would be enough explosives to make the two bombs and the other devices found with the brothers on Friday.

FBI agents found a "large pyrotechnic" in Dzhokhar's dormitory at the University of Massachusetts, according to an affidavit.

An unnamed police source told the Boston Globe that the victim of Friday's car-jacking, where the brothers held an SUV owner at gunpoint during their tense stand-off with police, has told police what Tamerlan said to him.

"We just killed a cop. We blew up the marathon. And now we're going to New York. Don't [expletive] with us," Tamerlan is reported to have told the victim. It was reported further that the brothers insisted that they needed the victim's bank card to fund their escape.

"If they had accomplices in New York, you'd think they would have had an established contingency plan to get down there to them, and wouldn't be shooting cops and car-jacking cars to steal ATM cards to finance their escape," the unnamed police source told the Globe.

"That said, we haven't ruled out anything in New York. We're looking into who they knew down there – and was anyone in New York prepared to hide them?"

Meanwhile, lawyers for Ms Russell said their client had no idea about the blasts.

"The reports of involvement by her husband and brother-in-law came as an absolute shock to all [the family]," a statement issued by the lawyers read.

"As a mother, a sister, a daughter, a wife, Katie deeply mourns the pain and loss to innocent victims — students, law enforcement, families and our community. In the aftermath of this tragedy, she, her daughter and her family are trying to come to terms with these events."

It appears Ms Russell has reverted to using her maiden name, after previously being known as Katherine Tsarnaeva.

Several sources have told US media that Dzhokhar – now in a "fair" condition in hospital after shooting himself in the throat – has made admissions to being involved in the blasts.

According to the Washington Post, Dzhokhar told FBI officials the motivation behind the attack was the US government's wars in Afghanistan and Iraq.

The newspaper's source was reported as saying the younger Tsarnaev brother has given "limited information", which suggested they acted without influence from overseas Islamist groups.

Dzhokhar has been charged with using a weapon of mass destruction and malicious destruction of property. He is expected face court again on May 30.

Adrian Lowe