Investigators have ruled out the Saudi national questioned in connection with yesterday’s Boston Marathon bombing as a suspect after a search of the man’s apartment and computer turned up nothing explosive- or terrorism-related, law-enforcement sources said.

The FBI-lead investigators are continuing to probe the man’s background but are also conducting a wider search of terrorism websites and chatter, surveillance footage, and the ball bearings and nails used in the two bombs, the sources said.

According to CNN, investigators have also found no link from the bombings to foreign or al-Qaeda connections.

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A roommate of the man described him as “a good boy,” incapable of such a monstrous attack.

Investigators early last night converged on a fifth-floor apartment where the person of interest lives with two roommates.

Mohammed Badawood, 20, described the man as “quiet and clean” and said he last saw him two days ago. Badawood told The Post he moved into the apartment about five months ago.

“He’s a good boy,” Badawood said of his roommate today. “I think he couldn’t do that.”

Officials showed up at the Revere apartment at about 5:30 p.m. yesterday in unmarked vehicles, a resident of the building said.

About an hour later, more vehicles, carrying agents of the FBI, Homeland Security and ATF also descended on the site, along with firefighters and a bomb squad.

Badawood said officials were searching his apartment when he arrived home last night at around 7.

Badawood said nothing was taken from the home and that officials told him the Saudi national was injured in the blast.

However, officials were later seen carrying bags out of the apartment complex. It is unclear if those items came from that apartment.

NYPD Commissioner Ray Kelly said today, “There are no specific threats against New York City but in the aftermath of the things that Boston’s experienced, we prepared as if yesterday was a prelude to an attack here in New York, in that, indeed, has been our SOP, our standard operating procedure since 9/11.”

Last night, Revere fire officials said they were called out to support bomb-squad officers as part of an investigation of a “person of interest” in the marathon attack.

By midnight, most of the authorities had left the complex, which sits on a piece of oceanfront property in the seaside city.

Investigators were looking for anything that might have been used set to off the devices, including a remote control, sources said.

Marcus Worthington, 24, a law student who lives in a neighboring building, said an ATF official told him investigators were responding to a tip about one of the apartments.

“He said that they were investigating a tip about a dangerous device in one of the apartments,” he said.

“I did ask him if it was a bomb or something, but he wouldn’t answer.”

Yesterday, police took the 20-year-old Saudi national into custody near the scene of yesterday’s horrific Boston Marathon bomb attack, law-enforcement sources told The Post.

The Saudi national was questioned by the FBI and local police at Boston’s Brigham and Women’s Hospital, where he was under heavy guard while being treated for shrapnel injuries to his leg.

He had suffered shrapnel wounds to the back of a leg but was expected to survive those injuries, a source said.

At the hospital, investigators seized the man’s clothes to examine whether they held any evidence that he was behind the attack. The law-enforcement sources also told The Post that the man was not free to leave the medical center.

As of last night, investigators had not yet directly asked the man whether he had set off the bombs. But they had asked him general questions, such as what he was doing in the area.

The man told police he had dinner Sunday night near Boston’s Prudential Center, about half a mile from the blast site, the sources said.

He also said that he went to the Copley Square area yesterday to witness the finish of the race.

The sources said that, after the man was grabbed by police, he smelled of gunpowder and declared, “I thought there would be a second bomb.”

He also asked: “Did anyone die?”

The twin blasts injured 176 people — 17 critically, authorities said today. The official death toll remained at three, but a law-enforcement source told The Post it could be as high as 12.

One witness told The New York Times there appeared to be 10 to 12 fatalities, including “women, children, finishers.” The wounds appeared to be “lower torso — the type of stuff you see from someone exploding out,” he said.

The dead included 8-year-old Martin Richard, whose mom and sister were hurt as they waited for his dad to finish running. Richard’s father, Bill, is a community leader in Dorchester.

Bomb-detecting cops swept the finish-line area twice yesterday morning — once early and again an hour before the first runners crossed, Boston police commissioner Ed Davis said.

“Those two EOD sweeps did not turn up any evidence,” Davis said.

But the city’s top cop said there was no way to prevent an attacker from coming and going, and perhaps planting explosives after police had swept the area.

“People can come and go and bring items in and out,” Davis said.

Police have not found any other explosives beyond the two that went off.

Richard DesLauriers, the FBI’s special agent in charge of the agency’s Boston office, vowed to go to the “ends of the earth” to hunt down the terrorists.

“This will be a worldwide investigation,” he said. “We will go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime and we will do everything we can to bring them to justice.”

Cops appealed to marathon spectators, asking for any still pictures or video shot in the neighborhood yesterday.

Even footage blocks away, well before or after the blast, might become crucial.

“Any information or photographs that happened — not just at that scene but anywhere in the immediate vicinity could be helpful to this investigation,” Davis said.

Additional reporting by David K. Li