Sixteen hours after investigators began interrogating him, the surviving suspect in the Boston Marathon bombings went silent: he'd just been read his constitutional rights.

Memorial in Copley Square to Marathon victims (Twitter via WCVB TV)

Dzhokhar Tsarnaev immediately stopped talking after a magistrate judge and a representative from the U.S. Attorney's office entered his hospital room and gave him his Miranda warning, according to four officials of both political parties briefed on the interrogation. They insisted on anonymity because the briefing was private.

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 that detonated pressure-cooker bombs at the marathon finish line, two U.S. officials said.

Officials also recovered a 9 mm handgun believed to have been used by Tamerlan from the site of an April 18 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 where Dzhokhar was hiding. 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."

But Kurt Schwartz, director of the Massachusetts Emergency Management Agency, said a police officer was shot within half a mile of where Tsarnaev was captured, "and I know who shot him."

Authorities had previously said Dzhokhar exchanged gunfire with them for more than an hour Friday night before they captured him inside a tarp-covered boat 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.