WASHINGTON – Federal law enforcement officials are sharpening their focus on the widow of the dead suspect in the Boston Marathon bombings after finding al-Qaida’s Inspire magazine and other radical Islamist material on her computer, according to law enforcement officials.

The probe of the computer belonging to Katherine Russell, 24, the widow of Tamerlan Tsarnaev, 26, is part of the effort by investigators to determine whether Russell knew anything about the April 15 bombing plot or helped the Tsarnaev brothers hide from authorities, according to the officials, who spoke on the condition of anonymity.

Dzhokhar Tsarnaev, 19, the surviving suspect, has told investigators that he and his brother learned to build the pressure-cooker bombs from English-language Inspire magazine and that they were partly influenced by the online sermons of Anwar al-Awlaki, the al-Qaida propagandist who was killed in a drone strike in Yemen in 2011.

According to officials, Dzhokhar Tsarnaev also told investigators that he and his brother built the bombs in Tamerlan Tsarnaev’s Cambridge apartment, where the elder brother lived with Russell and their daughter. Officials said Russell called her husband when she saw his photograph on television, following the FBI’s release of photos of the suspects after the bombings, but did not notify authorities.

Meanwhile, Newsday reported that a bathtub and kitchen sink in the apartment tested positive for residue from explosives. The testing was performed by federal explosives experts, and the results suggest bombs were either built in the bathroom and kitchen or stored there at some point, a law enforcement source told Newsday. One of the key questions for investigators is whether the radical Islamist materials on Russell’s computer belonged to her or were downloaded by her husband or someone else.

Russell’s attorney did not return phone calls seeking comment. The attorney, Amato DeLuca, has previously said his client played no role in the plot and was shocked to learn of the involvement of the Tsarnaev brothers.

Monday, FBI special agents spent about 90 minutes inside the North Kingstown, R.I., home of Russell’s parents. The agents left with bags of material and a sample of Russell’s DNA.

Two law enforcement officials said that investigators found fingerprints and female DNA on fragments of the pressure-cooker bombs that exploded at the marathon.

The DNA could have come from a woman who helped the suspects make the bombs or from a person who handled the materials at a store where the suspects bought them, said the officials. The DNA may have also come from someone in the crowd at the marathon, one of the officials said.

In another development, a funeral director in Worcester, Mass., said Tamerlan Tsaernav died from gunshot wounds and blunt trauma to his head and torso. Peter Stefan has Tsarnaev’s body and read details from his death certificate. The certificate cites Tsarnaev’s “gunshot wounds of torso and extremities” and lists the time of his death as 1:35 a.m. on April 19, Stefan said.

Tsarnaev died after a gunfight with authorities who had launched a massive manhunt for him and his brother. Police have said he ran out of ammunition before his younger brother dragged his body under a vehicle while fleeing,

Meanwhile, federal agents, state troopers and local law enforcement officers scoured a wooded area near Dartmouth, Mass., where Dzhokhar Tsarnaev attended college. Investigators are looking for possible evidence that might indicate the brothers tested explosives there, according to a law enforcement official. Residents in the vicinity had reported hearing loud noises coming from the woods March 30.

FBI spokesman Jason J. Pack said that the Dartmouth search by law enforcement and explosive-sniffing dogs is part of the continuing investigation into the bombings.

Law enforcement officials said several “persons of interest” in the United States and Russia are being investigated in connection with the brothers. One subject of primary focus remains the seven months that Tamerlan Tsarnaev spent in strife-torn regions of southern Russia in 2012.

During 16 hours of questioning in the hospital by the FBI, the younger Tsarnaev told agents that he and his brother initially considered carrying out suicide bombings and executing their plot on the Fourth of July at Boston’s large celebration along the Charles River, two law enforcement officials said.

But Tsarnaev said that he and his brother decided to launch their attack earlier because they were able to assemble the bombs in three or four days, more quickly than they had expected, according to the officials.

Officials have expressed skepticism about Tsarnaev’s account, saying that the complexity of the bombs makes it unlikely that the brothers could have completed assembling them as fast as he claimed.

According to a government document obtained by NBC News, a detailed analysis of the bombs used at the Boston Marathon – and the pipe bombs allegedly thrown at police from the Tsarnaevs’ car during a gunfight four days later – show striking similarities to instructions from Inspire magazine.

The report from the Terrorist Explosive Device Analytical Center said that the design of the pressure cooker bombs and the pipe bombs resembled the instructions provided in an article in the first issue of Inspire headlined “Make a Bomb in the Kitchen of Your Mom.”

The Boston pressure-cooker bombs use different triggers and power sources than the Inspire designs, but they match the instructions in the use of several components, including gunpowder from fireworks, according to the TEDAC analysis.

In another development, the administrator of a fund created to help the more than 260 people injured in the bombings and the families of the three who died in the attack plans to meet with victims next week with hopes of cutting the first checks by the end of June. Kenneth Feinberg is overseeing One Fund Boston, which has taken in more than $28 million as of Friday. The Boston Globe said the families of those who died and survivors who lost more than one limb in the attack could be in line to receive more than $1 million, while people who lost a single limb could get up to $1 million.

The Associated Press and Newsday contributed to this story.