Source: Dzhokhar Tsarnaev, Tamerlan Tsarnaev hatched Boston Marathon attack only about week before

BOSTON -- The accused marathon bomber told FBI interrogators that he and his brother had decided to bomb the race about a week before the deadly attack, according to a federal law enforcement source.

Dzhokhar Tsarnaev, 19, said that he and his brother, Tamerlan, 26, carried out the April 15 bombings to avenge perceived slights against Islam and because of America's wars in Iraq and Afghanistan, the source said.

Investigators continued Tuesday to piece together the brothers' lives and how far back the planning went. They learned that Tamerlan Tsarnaev in February purchased two reloadable mortar kits from a Phantom Fireworks shop in Seabrook, N.H., a company spokesman confirmed. Authorities have not said how the bombs were detonated and were investigating whether the mortar kits played a role.

The source said the construction of the bombs appears to follow step-by-step instructions found in al-Qaida's Western-aimed propaganda magazine, Inspire -- which includes the article "Make A Bomb in the kitchen of your Mom."

The magazine is the brainchild of Samir Khan, who became radicalized after 9/11 while an East Meadow school district high school student. He later moved to Yemen, where he was killed by an American drone in September 2011. Authorities have, so far, not uncovered evidence that the Tsarnaev brothers had ties to a foreign terror group, the source has said.

At a bedside court proceeding Monday, Tsarnaev was charged with conspiring to use a weapon of mass destruction and malicious destruction of property resulting in death. If convicted, he could face the death penalty.

Armed with how-to instructions found on the Internet, the brothers built homemade pressure-cooker bombs loaded with BBs and nails that were planted near the marathon's finish line, killing three people and injuring 260, according to authorities. On Tuesday, city officials revised the injury toll to that number from 176, saying some people hurt in the bombings went to hospitals by themselves and hadn't been included in the initial count.

Condition upgraded to fairTsarnaev remains hospitalized, recovering from what appears to be a botched suicide attempt, as authorities closed in on his hiding place Friday night in a boat in a suburban backyard. From his hospital bed, he is answering questions posed to him by an FBI team that specializes in interrogating terrorists. On Tuesday, his medical condition was upgraded to fair.

Tamerlan Tsarnaev was killed in a shootout with police early Friday morning. On Tuesday, the lawyer for his widow said his client is "doing everything she can to assist with the investigation" and knew nothing of the bombing plot.

Also, Ailina and Bella Tsarnaeva, the sisters of the bombing suspects, said in a statement that they are saddened to "see so many innocent people hurt after such a callous act."

The FBI has come under scrutiny for its handling of the elder brother's 2012 trip to Russia. Homeland Security Secretary Janet Napolitano said that her agency knew of that trip last year even though his name was misspelled on a travel document. Sen. Lindsey Graham (R-S.C.) said on Monday that the misspelling caused the FBI to miss the trip.

Napolitano said that, even though Tamerlan Tsarnaev's name was misspelled, redundancies in the system allowed his departure to be captured by U.S. authorities. She said that by the time he came back, an FBI alert on him had expired and his re-entry was not noted.

Russia alerted U.S.

The Boston Globe reported that in a closed briefing Tuesday, the Senate Intelligence Committee learned that Russia alerted the U.S. about Tamerlan Tsarnaev in "multiple contacts" - including "at least once since October 2011," said Richard Burr, a Republican of North Carolina, speaking with reporters afterward.

In the brothers' attempt to elude authorities, after authorities released surveillance pictures of them near the marathon's finish line, they are suspected of killing an MIT police officer in nearby Cambridge, carjacking a man in his Mercedes, and then leading police on a chase that culminated in the Watertown shootout.

The source said that investigators believe the brothers committed the carjacking and forced the driver to withdraw money from an ATM in order to fund a trip to New York. The unnamed carjacking victim said he fled the brothers when they stopped at a Cambridge gas station, according to a police report.

On Tuesday, several blocks near Boylston Street, where the bombs were planted, were reopened to traffic. Donations to the charity One Fund, set up to help victims, have reached $20 million, according to the Boston mayor's office. The figure includes $5 million from 50,000 people, along with several seven-figure donations from corporations.

Also, the city picked the fund's administrator: Ken Feinberg, the Washington, D.C., attorney who managed the compensation fund for 9/11 victims.

With Nicole Fuller and AP