ABBOTTABAD, Pakistan – Osama bin Laden made his final stand in a small Pakistani city where three army regiments with thousands of soldiers are based not far from the capital – a location that is increasing suspicions in Washington that Islamabad may have been sheltering him.

The U.S. acted alone in Monday’s helicopter raid, did not inform Pakistan until it was over and pointedly did not thank Pakistan at the end of a wildly successful operation. All this suggests more strain ahead in a relationship that was already suffering because of U.S. accusations that the Pakistanis are supporting Afghan militants and Pakistani anger over American drone attacks and spy activity.

Pakistani intelligence agencies are normally very sharp in sniffing out the presence of foreigners in small cities.

For years, Western intelligence had said bin Laden was most likely holed up in a cave along the Pakistan-Afghan border, a remote region of soaring mountains and thick forests where the Pakistan army has little presence. But the 10-year hunt for the world’s most-wanted man ended in a whitewashed, three-story house in a middle-class area of Abbottabad, a leafy resort city of 400,000 people nestled in pine-forested hills less than 35 miles from the national capital, Islamabad.

Sen. Carl Levin, the chairman of the Senate Armed Services Committee, said bin Laden’s location meant Pakistan had “a lot of explaining to do.”

“I think this tells us once again that unfortunately Pakistan at times is playing a double game,” said Sen. Susan Collins, R-Maine, a member of the Armed Services Committee.

A senior Pakistan intelligence official dismissed speculation that bin Laden was being protected.

“We don’t explain it. We just did not know — period,” he said, on condition his name not be released to the media.

Extra security forces swarmed the city on Monday, adding to Abbottabad’s already massive military presence.

Associated Press reporters saw the wreckage of one of the American helicopters that malfunctioned and had to be destroyed during the operation. Residents described the sounds of bullets, the clatter of chopper blades and two large explosions as the raid went down.

It was unclear how long bin Laden had been holed up in the house with members of his family. From the outside, the house resembled many others in Pakistan and even had a flag flying from a pole in the garden, apparently a Pakistani one. It had high, barbed-wire topped walls, few windows and was located in a neighborhood of smaller houses, shops, dusty litter-lined streets and plots used for growing vegetables.

Neighbors said large Landcruisers and other expensive cars were seen driving into the compound, but they had no indication that foreigners were living inside. Salman Riaz, a film actor, said that five months ago he and a crew tried to do some filming next to the house, but were told to stop by two men who came out.

“They told me that this is haram (forbidden in Islam),” he said.

A video aired by ABC News that purported to show the inside of the compound included footage of disheveled bedrooms with floors stained with large pools of blood and littered with clothes and paper.

After nightfall on Monday, a single light shone from inside the compound.

Some residents were alarmed. “We’re very concerned for this town. It was a very safe place. Now there could be al-Qaida everywhere,” said Naeem Munir.

The compound, which an Obama administration official said was “custom built to hide someone of significance,” was about a half-mile away from the Kakul Military Academy, one of several military installations in the bustling, hill-ringed town.

“Personally I feel that he must have thought it was the safest area,” said Asad Munir, a former station chief of Pakistan’s Inter-Services Intelligence agency, or ISI, in the northwest. “Abbottabad is a place no one would expect him to live.”

Suspicions that Pakistan harbors militants have been a major source of mistrust between the CIA and Pakistan’s ISI, though the two agencies have cooperated in the arrests of al-Qaida leaders since the Sept. 11, 2001 attacks, including several in towns and cities outside the border area.

“Why had Pakistan not spotted he is living in a nice tourist resort just outside Islamabad?” asked Gareth Price, a researcher at Chatham House think-tank in London. “It seems he was being protected by Pakistan. If that is the case, this will be hard for the two sides to carry on working together. Unless Pakistan can explain why they didn’t know, it makes relations difficult.”

Relations between Pakistan’s main intelligence agency and the CIA had been very strained in recent months after a CIA contractor shot and killed two Pakistanis in January, bringing Pakistani grievances out into the open. Since then, a Pakistani official has said that joint operations had been stopped, and that the agency was demanding the Americans cut down on drone strikes in the border area.

The U.S. has fired hundreds of drones into the border regions since 2008, taking out senior al-Qaida leaders in a tactic seen by many in Washington as vital to keeping the militant network and allied groups living in safe havens on the back foot.

While tensions may run high, it is unlikely that either nation could afford to sever the link completely. Pakistan has nuclear weapons, and the U.S. needs Islamabad to begin its withdrawal from Afghanistan this year as planned. Pakistan relies heavily on the United States for military and civilian aid.

Some of the strongest allegations about ISI involvement in sheltering bin Laden were made in Afghanistan, where President Hamid Karzai has repeatedly said that more of the American focus should be across the border in Pakistan.

“For years we have said that the fight against terrorism is not in Afghan villages and houses,” said Karzai. “It is in safe havens, and today that was shown to be true.”

There was no evidence of direct ISI collusion, and American officials did not make any such allegations.

“There are a lot of people within the Pakistan government, and I am not going to speculate about who, or if any of them had foreknowledge about bin Laden being in Abbottabad but certainly its location there . . . raises questions,” said White House counterterrorism adviser John Brennan.

Some analysts suggested that Pakistan would have little interest in sheltering bin Laden. They contrasted the al-Qaida leader with Afghan Taliban leaders, whom Pakistan views as useful allies in Afghanistan once America withdraws. Al-Qaida has carried out scores of attacks inside Pakistan in recent years.

Last month, Adm. Mike Mullen, the chairman of the U.S. joint chiefs of staff, accused Pakistan’s military-run spy service of maintaining links with the Haqqani network, a major Afghan Taliban faction.

Pakistan’s government and army are very sensitive to concerns that they are working under the orders of America and allowing U.S. forces to operate here.