Despite the enormous efforts Russia has undertaken to keep the peace at the Olympic games beginning tomorrow in Sochi, security experts say it may not be enough to prevent an attack—likely from militants already inside the city and hiding within the massive labor force hired to support the games.

“The Russians made a big mistake; they announced well in advance when they would close the ring of steel,” said Bill Rathburn, director of security for the 1996 Olympics in Atlanta. That may have bought militants time to enter the city and begin preparing for an attack ahead of the security crackdown, he said.

Don Borelli, a veteran FBI agent and former assistant special agent in charge of the New York Joint Terrorism Task Force, also said that it’s those who are already inside who may pose the greatest threat, pointing to the workers hired for hospitality and service jobs.

“It takes thousands for this,” Borelli said, “Do they know who they’ve hired?”

Russia’s operations to clear the city of Sochi and cut it off from the volatile insurgency in the surrounding Northern Caucuses region, perhaps the largest security undertaking in Olympic history—have been a massive and brutal enterprise, with months of large-scale arrests and crackdowns on militants, ethnic Muslim communities, and political dissidents leading up to the Games. When they begin, Russia will have over 40,000 security forces inside the city of Sochi and thousands more special operations military units stationed around it to cut off infiltration routes from the surrounding mountains.

But as Russia’s longstanding wars against hostile breakaway populations have shown, even the largest military campaign can’t fully eliminate the threat from groups with active grievances and a willingness to use indiscriminate violence, a threat Borelli said is potentially compounded by the lack of cooperation and intelligence sharing between Russian security services and other participating countries like the U.S.

“I fully believe the Russians had a lot more information they could have shared that might have prevented the Boston bombing,” he said.

Even before the opening ceremony Friday morning, terrorist attacks targeting the games have already claimed lives. In December 2013, two bomb attacks carried out a day apart on Volgograd’s public transit system, killed 32 people and injured more than 85. The suspected mastermind of the bombings was killed Wednesday in Dagestan, 400 miles northeast of Sochi, after a shootout with Russian police in an operation that also led to the arrest of two brothers accused of aiding the Volgograd bombers.

The militant jihadist group responsible for the Volgograd bombing has not yet responded publicly to the killing and arrest of its members, but earlier threatened an attack at the Olympics and warned President Putin to “expect a present” at the Sochi games.

Despite the Volgograd bombings, the restive insurgency, and the continued threats of violence, the head of Russia’s Olympic security, Alexey Lavrishev, announced at a press conference last week that “as of today, there are no real and actual threats and nemeses to the athletes and spectators during the Games.” Today, Russian Deputy Prime Minister Dmitry Kozak, went even further, stating “We can guarantee the safety of people as well as any other government hosting any mass event.”

Attacks on Olympic athletes and at sporting events, where security protocols are strictest and forces are most concentrated, would be hardest to pull off according to most experts.

Though Rathburn said “terrorists will attempt to stage a spectacular attack” to disrupt the games and capture media attention, he said that was less likely to succeed. “The least there will be," Rathburn believes, "is a harassment type incident,” involving a bomb or small arms attack at the security perimeter outside of the tightly controlled Olympic village.

The “soft targets” most vulnerable to attacks are those outside of the tightly controlled Olympic village, like transportation lines moving large groups of people and “fan zone” areas in hotels and bars where people gather to watch the games, both experts agreed.

“I would be afraid to use public transportation in Sochi during the games,” Rathburn said.

“The venues themselves are hard targets,” Borelli said, assessing the threat to Olympic athletes as “very low,” while adding that a lone wolf who could keep a low profile by operating independently of larger terrorist networks would be nearly impossible to intercept before they tried an attack.

“It doesn’t matter if you’re in Sochi, New York or Kansas, you’d need a crystal ball to stop that,” Borelli said.