Investigators believe Tamerlan Tsarnaev and his younger brother, Dzhokhar, likely made the bombs they are suspected of setting off at last month’s Boston Marathon in Tamerlan’s home in Cambridge, Massachusetts, law enforcement officials said on Friday.

FBI agents have been questioning Tamerlan’s wife, Katherine Russell, and other witnesses for days to try to piece together exactly how and where the devices were made and what people knew about the brothers’ beliefs and plans.

Investigators said they are increasingly convinced that the ethnic Chechen brothers lifted their bomb designs from Internet postings by Islamic militants, though they substituted some components, and made the lethal devices less than five miles from the spot where the bombs killed three and injured 264.

Tamerlan Tsarnaev, 26, and his 24-year-old wife lived on Norfolk Street in Cambridge. Tamerlan spent hours alone there, minding the couple’s young child as his wife worked up to 80 hours a week as a health aide, her lawyer, Amato DeLuca, has said. His brother Dzhokhar, 19, was enrolled as a student at the University of Massachusetts and lived in a dormitory on the school’s Dartmouth campus, about an hour’s drive from Boston.

Investigators believe that during his long days at home alone, Tamerlan, who enjoyed expensive clothes and cars and occasionally worked as a mechanic, likely honed his bomb-making skills with materials found around the house. Police had said the bombs were built from pressure cookers packed with nails and ball bearings.

Soon after investigators released pictures of the suspected bombers and asked the public for help in identifying and locating the two men, Tamerlan communicated with his wife, said officials familiar with the investigation, who asked for anonymity when discussing sensitive information.

Law enforcement officials said the pair communicated after she saw the pictures of her husband and brother-in-law on television. Russell sent a text message to her husband, a national security source said.

Russell’s lawyer declined to comment on Friday. He said earlier in the week that Russell spent many hours talking with officials and was eager to “provide as much assistance to the investigation as she can.”

Officials took DNA samples from her after a woman’s DNA was found on one of the bomb remnants. Other media have reported that the DNA on the bomb is not a match to Russell. The FBI declined to comment on the matter.

OUT OF SIGHT

Ever since her husband was killed after a shootout with police, Russell, still dressing in long skirts and traditional Muslim head scarf, has been trying to stay out of sight while living with her parents in North Kingstown, Rhode Island. She has tried to distance herself from the man she married in June, 2010 at a mosque in Boston’s Dorchester neighborhood.

She declined to pick up Tamerlan Tsarnaev’s body from the Massachusetts Medical Examiner’s office, allowing his relatives to claim the remains and arrange for a funeral.

Tsarnaev’s death was ruled a homicide and he died from gunshot wounds and blunt trauma to the head and torso, according to a person familiar with the death certificate. Dzhokhar drove over Tamerlan while trying to escape as police shot at the pair.

Meanwhile investigators were also pursuing leads across the state and focusing their attention on the Dartmouth area where Dzhokhar Tsarnaev, the surviving suspect, was a student.

“The searches at various locations in Dartmouth, Mass., today are part of the ongoing investigation into the marathon bombing,” a spokeswoman for the U.S. Attorney’s office said. “Residents should be advised that there is no threat to public safety.”

Earlier in the week, authorities arrested three of Dzhokhar Tsarnaev’s college friends, charging two with hiding evidence and one with lying to investigators.

Two Russian-speaking students from Kazakhstan, Dias Kadyrbayev and Azamat Tazhayakov, were accused of removing Tsarnaev’s backpack filled with empty fireworks and his laptop from his dormitory room, the government said in court documents.

Investigators found the backpack in a local landfill. A third college friend, Robel Phillipos, an American, lied to investigators about what the trio had done, authorities said.

All are now being held in jail.

In Kazakhstan, Dias Kadyrbayev’s father, Murat, said he was certain his son was innocent and that the students “are assisting the investigation in every possible way.”

He described how his son was taken into custody in the hours after a shoot-out between the Tsarnaev brothers and police.

“The last time I spoke with him after the arrest, he told me then over the phone, ‘Papa, they’ve arrested us like in an American movie. They came … with a platoon of soldiers, with submachine guns, with laser sights,’” the father said.

U.S. officials said the two Kazakhs were initially picked up on immigration violations last month and were later charged with obstruction of justice.

The mayor of Boston expressed anger that officials had disclosed that the Tsarnaev brothers initially planned their attacks for July 4. “It frightens people. We continue to get information out there that will arouse people,” Mayor Thomas Menino said at a luncheon. The city will hold its annual Fourth of July celebrations, which include a concert by the Boston Pops Orchestra and fireworks, as planned.

“We won’t be frozen by terrorists,” he said.

(Reuters)