The 19-year-old suspect in the Boston Marathon bombings has told interrogators that the American wars in Iraq and Afghanistan motivated him and his brother to carry out the attack, according to U.S. officials familiar with the interviews.

From his hospital bed, where he is now listed in fair condition, Dzhokhar Tsarnaev has acknowledged his role in planting the explosives near the marathon finish line on April 15, the officials said. The first successful large-scale bombing in the post-Sept. 11, 2001, era, the Boston attack killed three people and wounded more than 250 others.

The officials, who spoke on condition of anonymity to describe an ongoing investigation, said Dzhokhar and his older brother, Tamerlan Tsarnaev, who was killed by police as the two attempted to avoid capture, do not appear to have been directed by a foreign terrorist organization.

Rather, the officials said, the evidence so far suggests they were “self-radicalized” through Internet sites and U.S. actions in the Muslim world. Dzhokhar Tsarnaev has specifically cited the U.S. war in Iraq, which ended in December 2011 with the removal of the last American forces, and the war in Afghanistan, where President Obama plans to end combat operations by the end of 2014.

Obama has made repairing U.S. relations with the Islamic world a foreign policy priority, even as he has expanded drone operations in Pakistan and other countries, which has inflamed Muslim public opinion.

See the names and stories of the Boston Marathon victims

Dzhokhar Tsarnaev has provided limited information to authorities that indicates he and his brother acted independently, without direction or significant influence from Islamist militants overseas. U.S. officials said they are still working to assemble a detailed timeline of a trip the older Tsarnaev took to Russia, but see no evidence that he received instructions there that led to the attack.

“These are persons operating inside the United States without a nexus” to an overseas group, a U.S. intelligence official said.

U.S. officials have said that the FBI questioned Tamerlan Tsarnaev at the behest of Russian authorities who had become concerned that he was becoming radicalized. The request was conveyed to officials at the U.S. Embassy in Moscow. U.S. officials said they sought follow-up information from Russia, but that Moscow failed to respond.

Officials also expressed skepticism that Russian authorities were concerned about the elder Tsarnaev’s contacts during his trip to Russia. “The evidence points to the fact that they let him into the country and let him out of the country,” the U.S. official said. “They didn’t take any legal action, which they could have while he was there.”

The Boston Public Health Commission said Tuesday that at least 250 people have sought medical treatment at area hospitals for injuries related to the bombings. The number has gradually increased in the past week as people seek delayed help for minor injuries that didn’t heal on their own or hearing problems, according to a spokeswoman for the health commission.

The U.S. Attorney’s Office in Boston said Tuesday that according to Beth Israel Deaconess Medical Center, Dzhokhar Tsarnaev’s condition is now listed as “fair.” The office said it was releasing the information at the request of the hospital, where Tsarnaev is being treated.

Dzhokhar Tsarnaev was charged Monday with using a weapon of mass destruction and malicious destruction of property, counts that could carry the death penalty if convicted. He made his first court appearance in an unusual nonpublic proceeding in which a federal judge and several lawyers went to his hospital bed in Boston.

The youngest of those killed in the blasts, 8-year-old Martin Richard, was buried Tuesday morning in a private ceremony, his parents said.

“A private Funeral Mass was celebrated this morning with immediate family,” Denise and Bill Richard said in a statement. “We laid our son Martin to rest, and he is now at peace. We plan to have a public memorial service in the coming weeks to allow friends and loved ones from our community to join us for a celebration of Martin’s life.” The couple expressed thanks for a “tremendous” outpouring of love and support over the past week.

On Capitol Hill, members of the Senate Intelligence Committee grilled FBI Deputy Director Sean Joyce for more than two hours Tuesday on details of the ongoing investigation.

Sen. Dianne Feinstein (D-Calif.), who chairs the committee, said federal investigators face “a long, arduous task” in reconstructing the events that led to the bombing. But she said she has “complete confidence that this will be completely solved.”

The panel’s top Republican, Sen. Saxby Chambliss (Ga.), said the surviving suspect has provided “minimal information” to investigators, and he suggested that miscommunications may have reemerged at agencies responsible for counterterrorism following reforms in the wake of the Sept. 11, 2001, attacks.

“I think there’s been some stovepipes reconstructed that were probably unintentional,” Chambliss told reporters. “But we’ve got to review that again and make sure that there is the free flow of information.”

Sen. Susan Collins (R-Maine) also said she was “very concerned that there still seem to be serious problems with sharing information, including critical investigative information.” She said it was “troubling to me that this many years after the attacks on our country in 2001 that we still seem to have stovepipes that prevent information from being shared effectively, not only among agencies but also within the same agency in one case.”

The criminal complaint against Tsarnaev, filed in U.S. District Court in Boston, ended a debate over how the case should be handled. Some congressional Republicans had insisted that Tsarnaev be designated an “enemy combatant,’’ which would enable the government to charge him under the laws of war in a military commission or to hold him indefinitely.

White House press secretary Jay Carney rejected that approach Monday, saying that the suspect cannot be tried in a military commission under federal law because he is a U.S. citizen. “We will prosecute this terrorist through our civilian system of justice,’’ Carney said at a White House news briefing.

Three defense lawyers, along with two federal prosecutors, joined Tsarnaev for the court session at his hospital bed, where the suspect is recuperating from gunshot wounds to the head, neck, legs and hands, according to a transcript provided by the court.

Federal Magistrate Judge Marianne B. Bowler advised Tsarnaev of his rights and the charges against him, the transcript said.

“How are you feeling?” a doctor, identified as Dr. Odom, asked. “Are you able to answer some questions?” Tsarnaev “nods affirmatively,’’ said the transcript, which added that the only word he spoke during the hearing was “no,” when asked whether he could afford a lawyer. William Fick, one of the defense lawyers, said he would reserve questions about bail and other matters, according to the transcript. The judge then ended the session, saying she found the defendant “alert, mentally competent, and lucid.”

An affidavit by FBI agent Daniel R. Genck that accompanied the complaint portrayed Tsarnaev as a relatively low-tech operative who was caught on the day of the marathon on multiple surveillance cameras. Genck described how Tsarnaev and his brother were captured on camera walking near the finish line April 15, both carrying large knapsacks. They stood together for several minutes, appearing to watch the race.

Tamerlan broke off and walked toward where the first improvised explosive device would soon detonate, Genck said. Three minutes later, Dzhokhar walked in the same direction, then stopped and slipped his knapsack onto the ground. He then stood looking at his cellphone, and even appeared to snap a picture with it.

About 30 seconds before the first explosion, Genck’s affidavit said, Dzhokhar, standing in front of a restaurant, lifted the phone to his ear as though he were speaking and held it there. The first bomb exploded. “Virtually every head turns to the east [toward the finish line] and stares in that direction in apparent bewilderment and alarm,’’ Genck said.

Dzhokhar, referred to in the complaint as “Bomber Two,’’ stood out. “[V]irtually alone among the individuals in front of the restaurant,’’ the affidavit said, he “appears calm.’’

He then rapidly walked away from the direction of the first explosion, his knapsack on the ground.

About 10 seconds later, the second bomb exploded where his knapsack had been, Genck said.

The document also provides details about the explosives used in the attack, saying they were “low-grade” and housed in pressure cookers that contained metallic BB’s and nails. Many of the BB’s were encased within an adhesive material, the complaint said, and several explosives discovered in a car the brothers used and at the scene of the shootout with police in Watertown, Mass., were similar.

Authorities were continuing a worldwide investigation of the marathon attack and looking into whether foreign or domestic terrorist groups helped. No evidence of such a connection has emerged, law enforcement officials said.

Authorities are trying to trace a handgun recovered from the suspects. Law enforcement sources said the effort has been delayed because the serial number was removed. Technicians are working to determine the numbers, after which the weapon will be traced by a Bureau of Alcohol, Tobacco, Firearms and Explosives facility in West Virginia.

New information also emerged about Tamerlan’s earlier brushes with the law.

Authorities in Massachusetts confirmed that they are investigating whether the elder brother may have been connected to a triple homicide in Waltham, Mass., in September 2011, prosecutors said. The Boston Globe reported that Tamerlan was friends with Brendan Mess, 25, one of three people found dead in an apartment on the afternoon of Sept. 12.

MaryBeth Long, a spokeswoman for the Middlesex County District Attorney’s Office, told The Washington Post that authorities “will review any new information that may come to light in that case as a result of the investigation” of the marathon bombing.

Court records in Cambridge, Mass., meanwhile, showed that Tamerlan was arrested on charges of assault and battery in July 2009 after slapping a girlfriend. The incident occurred at his apartment on Norfolk Street after an argument between him and Nadine Ascencao over another woman, according to the incident report. The records indicate that the case was dismissed about six months later.

Jenna Johnson in Boston. Kathy Lally and Will Englund in Moscow and Ed O’Keefe, Jerry Markon, William Branigin, Greg Jaffe, Julie Tate and Joel Achenbach in Washington contributed to this report.