"We know some of them got through the perimeter," Rep. Michael McCaul, chairman of the U.S. House Homeland Security Committee, told The Associated Press. "She's for real. What we don't know is how many more black widows are out there."
Russian authorities have blamed the so-called "black widows" of slain insurgents for previous suicide attacks in the country.
The Black Sea resort town will host the games amid concerns about security and potential terrorist attacks.
The southern city of Volgograd was rocked by two suicide bombings in late December, which killed 34 and injured scores more. An Islamic militant group in Dagestan posted a video on Sunday claiming responsibility for the bombings and threatened to strike the games in Sochi, about 500 kilometers (300 miles) west of Dagestan.
McCaul, a Republican from Texas, said he had numerous meetings with officials in Moscow and Sochi, and was briefed by the joint operation center in Sochi, which is responsible for overall security in the area.
"The one improvement I would ask of the Russians is to allow our intelligence services to coordinate and cooperate better with theirs," McCaul said. Although the Russian side was confident that it could provide security, the U.S. has information that could help keep the games safe, he said.
The congressman also expressed concern that terrorists could have gotten into Sochi before security was tightened.
"How many potential cells could be in Sochi and the Olympic village?" he said. "But after 'the ring of steel' was implemented we have this one person who seems to have been able to penetrate it. It does demonstrate vulnerability."
Police material distributed to the hotel staff included pictures of two other women in veils: 26-year-old Zaira Aliyeva and 34-year-old Dzhannet Tsakhayeva. It said they had been trained "to perpetrate acts of terrorism."
It warned that the two women "are probably among us," but, unlike Ibragimova's case, did not say if they are in Sochi.
No further information was provided about the two women or their motivation. The term "black widow" refers to the belief that women who have carried out past suicide attacks in Russia did so to avenge the deaths of husbands or other male relatives.
Security officials in Sochi were not available for comment on Tuesday.
The Olympics are to be held Feb. 7-23. Russia has mounted an intense security operation in the city, but concern persists that "soft targets" outside the Olympic venues, such as buses and tourist facilities, are vulnerable to attack.
Russian troops also have been active fighting militants in Dagestan, one of the predominantly Muslim republics in Russia's North Caucasus and the center of an Islamic insurgency that has engulfed the region.
On Tuesday, troops shot dead the leader of a militant group, Interior Ministry spokeswoman Fatina Ubaidatova said. She said the militant, Eldar Magatov, was wanted in attacks on security forces, bombings and the extortion of businessmen.
Interior Ministry troops elsewhere in Dagestan defused an explosive device placed near a village administration building and engaged in a firefight with militants holed up in a house, the spokeswoman said.
Associated Press writer Arsen Mollayev in Makhachkala, Russia, contributed to this report.