The two brothers accused of setting off bombs at the Boston Marathon are believed to have acted entirely on their own, using instructions for bomb making from an online magazine. NBC's Pete Williams reports.

A preliminary examination of the cellphones and computers used by the Tsarnaev brothers has found no indication of an accomplice in the Boston Marathon bombing, according to a U.S. counter-terrorism source briefed on the FBI investigation.

The source stressed that the investigation is ongoing, but bureau officials at this point appear increasingly confident that “nobody else was involved,” said the source.

Dzhokhar Tsarnaev has told investigators he and his brother acted alone, learned to build the pressure-cooker bombs over the Internet and were motivated by a desire to defend Islam because of "the U.S. wars in Iraq and Afghanistan," said the source, who has received multiple briefings on the probe.

Dzhokhar Tsarnaev has also told investigators that he and his brother got instructions on building bombs from an online magazine published by al Qaeda, federal law enforcement officials told NBC News.

He told investigators that the brothers read the instructions in Inspire, an online, English-language magazine that terror monitoring groups say al Qaeda began publishing in 2010.

The magazine has twice included articles on building bombs with kitchen pressure cookers — the method investigators say Tsarnaev and his brother, Tamerlan, used in the Boston attack.

Dzhokhar Tsarnaev was arrested Friday after a manhunt that shut down Boston and its suburbs. He was interrogated in a Boston hospital bed, where he is recovering from injuries sustained in shootouts during the hunt. His condition was upgraded Tuesday to fair from serious, federal prosecutors in Massachusetts said.

Federal authorities charged him Monday with using a weapon of mass destruction, which could carry the death penalty. State officials said they expected to charge him in the death of a campus patrol officer as part of the shootout that authorities say the brothers carried out early Friday, NBC affiliate WHDH reported.

Dzhokhar Tsarnaev told investigators that he and Tamerlan, who was killed after the shootout with police in the Boston suburb of Watertown, were motivated by religious fervor but were not in touch with overseas terrorists or terror groups, officials said.

Several officials familiar with the interrogation of Dzhokhar Tsarnaev described his behavior as cooperative.

A fireworks store in Seabrook, N.H., confirmed Tuesday that the older brother bought two large pyrotechnic kits there Feb. 6.

Tamerlan Tsarnaev bought two “good-size” mortar kits, consisting of tubes and shells, and black powder, said William Weimer, vice president of the store, Phantom. He said Tsarnaev paid $199.99 under a buy-one-get-one-free deal.

Weimer said he had Tsarnaev’s driver’s license on file, and said the FBI was at the store Friday or Saturday.

From his hospital room, the younger brother, in what was officially his first court appearance, communicated mostly by shaking his head. He spoke once, when asked whether he could afford a lawyer. He said, “No.” He was assigned three federal public defenders.

The White House said Monday that Tsarnaev will be tried in civilian court. Some Republicans have called for him to be treated as an enemy combatant, and tried in a military commission.

Tsarnaev, 19, is a naturalized American citizen of Chechen origin. White House press secretary Jay Carney said that American citizens cannot under law be tried in military commissions. He also noted that “hundreds of terrorists” have been convicted and imprisoned since Sept. 11, 2001, under the civilian court system.

In the hospital, Tsarnaev was advised of his rights and charged with one count of using and conspiring to use a weapon of mass destruction in the U.S. and one count of malicious destruction of property with an explosive device.

The suspect agreed to “voluntary detention” but declined to answer questions about bail, according to a court record. A probable cause hearing was set for May 30.

The twin blasts near the marathon finish line killed three people and injured more than 170. On Tuesday, there were still 45 victims in Boston hospitals, one in critical condition. Boston Children’s Hospital said that a 7-year-old girl with leg injuries had been upgraded to serious condition from critical.

Investigators want to speak with Tamerlan’s wife, Katherine Russell Tsarnaev, who converted to Islam after she met her future husband at a nightclub. She dropped out of college, got married and had a baby three years ago.

Her lawyer told The Associated Press that he was trying to work out an interview. He said his client worked up to 80 hours a week as a home health aide while Tamerlan watched their daughter. He said she did not suspect he was plotting something.

He said she last saw her husband at home on Thursday morning, hours before he and his younger brother allegedly executed the campus police officer, carjacked an SUV and led police on a wild bomb-tossing chase that ended in a 200-bullet gunbattle.

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