BOSTON (AP) — The surviving suspect in the Boston Marathon bombings acknowledged to the FBI his role in the attacks but did so before he was advised of his constitutional right to keep quiet and seek a lawyer, U.S. officials said Wednesday.

Once Dzhokhar Tsarnaev was read his rights on Monday, he immediately stopped talking, according to four officials of both political parties who were briefed on the interrogation but insisted on anonymity because the briefing was private.

After roughly 16 hours of questioning, investigators were surprised when a magistrate judge and a representative from the U.S. Attorney's office entered the hospital room and read Tsarnaev his rights, the four officials and one law enforcement official said. Investigators had planned to keep questioning him.

It is unclear whether any of this will matter in court since the FBI says Tsarnaev confessed to a witness and U.S. officials said Wednesday that physical evidence, including a 9 mm handgun and pieces of a remote-control device commonly used in toys, was recovered from the scene.

But the debate over whether suspected terrorists should be read their Miranda rights has become a major sticking point in the debate over how best to fight terrorism. Many Republicans, in particular, believe Miranda warnings are designed to build court cases, and only hinder intelligence gathering.

Christina DiIorio Sterling, a spokeswoman for U.S. Attorney Carmen Ortiz, said in an email, "This remains an ongoing investigation and we don't have any further comment."

Before being advised of his rights, the 19-year-old suspect told authorities that his older brother, Tamerlan Tsarnaev, 26, only recently had recruited him to be part of the attack, two U.S. officials said.

The CIA, however, named Tamerlan to a terrorist database 18 months ago, officials said Wednesday, an acknowledgment that will undoubtedly prompt congressional inquiry about whether investigators took warnings from Russian intelligence officials seriously enough.

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The U.S. officials who discussed the terrorist database and other details of the investigation are in addition to those who discussed the Miranda warning. They were close to the investigation and insisted on anonymity because they were not authorized to discuss the case with reporters.

Tamerlan Tsarnaev, whom authorities have described as the driving force behind the plot, was killed in a shootout with police. Dzhokhar is recovering in a hospital from injuries suffered during a getaway attempt.

Authorities had previously said Dzhokhar exchanged gunfire with them for more than an hour Friday night before they captured him inside a boat covered by a tarp in a suburban Boston neighborhood backyard. But two U.S. officials said Wednesday that he was unarmed when captured, raising questions about the gunfire and how he was injured.

More than 4,000 mourners at the Massachusetts Institute of Technology paid tribute to a campus police officer who authorities say was gunned down by the bombing suspects.

Among the speakers in Cambridge, just outside Boston, was Vice President Joe Biden, who condemned the bombing suspects as "two twisted, perverted, cowardly, knockoff jihadis."

Investigators have said the brothers appeared to have been radicalized through jihadist materials on the Internet and have found no evidence tying them to a terrorist group.

Dzhokhar told the FBI that they were angry about the U.S. wars in Afghanistan and Iraq and the killing of Muslims there, officials said.

Dzhokhar's public defender had no comment on the matter Wednesday. His father has called him a "true angel," and an aunt has insisted he's not guilty.

Investigators have found pieces of remote-control equipment among the debris and were analyzing them, officials said. One official described the detonator as "close-controlled," meaning it had to be triggered within several blocks of the bombs.

That evidence could be key to the court case. And an FBI affidavit said one of the brothers told a carjacking victim during their getaway attempt, "Did you hear about the Boston explosion? I did that."

Officials also recovered a 9 mm handgun believed to have been used by Tamerlan from the site of a Thursday night gunbattle that injured a Massachusetts Bay Transportation Authority officer, two U.S. officials said.

The officials told the AP that no gun was found in the boat. Boston police Commissioner Ed Davis said earlier that shots were fired from inside the boat.

Asked whether the suspect had a gun in the boat, Davis said, "I'm not going to talk about that."

Kurt Schwartz, director of the Massachusetts Emergency Management Agency, did respond to the report.

"Within half a mile of where this person was captured, a police officer was shot. And I know who shot him." Schwartz said. "And there were three bombs that went off, and I know where those bombs came from. ... To me, it does not change anything. This guy was captured alive and will survive. True or not true, it doesn't change anything for me."

The suspects' parents, Anzor Tsarnaev and Zubeidat Tsarnaeva, plan to fly to the U.S. from Russia on Thursday, the father was quoted as telling the Russian state news agency RIA Novosti. The family has said it wants to take Tamerlan's body back to Russia.

In Russia, U.S. investigators traveled to the predominantly Muslim province of Dagestan and were in contact with the brothers' parents, hoping to gain more information.

Investigators are looking into whether Tamerlan, who spent six months in Russia's turbulent Caucasus region in 2012, was influenced by the religious extremists who have waged an insurgency against Russian forces in the area for years. The brothers have roots in Dagestan and neighboring Chechnya but had lived in the U.S. for about a decade.

While in the U.S., the brothers received welfare benefits.

The Office of Health and Human Services in Massachusetts confirmed a Boston Herald report Wednesday that Tamerlan, his wife and daughter had received welfare benefits up until last year, when he became ineligible based on family income.

The state also says Tamerlan and his brother received welfare benefits as children through their parents while the family lived in Massachusetts.

Neither was receiving benefits at the time of the bombing.

At MIT, bagpipes wailed as students, faculty and staff members and throngs of law enforcement officials paid their respects to MIT police Officer Sean Collier, who was ambushed in his cruiser three days after the bombing.

Biden told the Collier family that no child should die before his or her parents, but that, in time, the grief will lose some of its sting.

"The moment will come when the memory of Sean is triggered and you know it's going to be OK," Biden said. "When the first instinct is to get a smile on your lips before a tear to your eye."

The vice president also sounded a defiant note.

"The purpose of terror is to instill fear," he said. "You saw none of it here in Boston. Boston, you sent a powerful message to the world."

In another milestone in Boston's recovery, the area around the marathon finish line was reopened to the public, with fresh cement still drying on the repaired sidewalk. Delivery trucks made their way down Boylston Street under a heavy police presence, though some damaged stores were still closed.

"I don't think there's going to be a sense of normalcy for a while," Tom Champoux, who works nearby, said as he pointed to the boarded-up windows. "There are scars here that will be with us for a long time."

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Jakes and Dozier reported from Washington. Associated Press writers David Crary, Bridget Murphy and Bob Salsberg in Boston, Lynn Berry in Moscow and Matt Apuzzo, Adam Goldman, Eric Tucker, Pete Yost and Eileen Sullivan in Washington contributed to this report.