WASHINGTON (CNN) -- Investigators are trying to determine if anyone involved in an alleged terror plot against the New York City subway system is in the United States, the city's top police official said Sunday.New York City Police Commissioner Ray Kelly told CNN's "Late Edition" that authorities have been looking into reports that a suspect might be in the country."We have information that indicated perhaps there were people here, but it just didn't have a sufficient description ... for us to put that information out," he said. FBI and Department of Homeland Security officials told CNN Friday they have been unable to verify that claim -- or if the person even exists. Kelly said there were ongoing interrogations in Iraq and he thought investigators would soon learn if there was "real substance" to the threat.The terror threat originated in Iraq and involved the use of explosives hidden in bags or baby strollers, officials familiar with the investigation have told CNN.The tipster in Iraq failed some sections of the polygraph test, but passed the section pertaining to the information about the New York threat, an official said. The source, who previously had given reliable information, tipped authorities to a plot involving 15 to 20 people, one official said.That information, sources said, led to a military operation Wednesday night in Musayyib, about 45 miles south of Baghdad, where, military officials said, three al Qaeda suspects were arrested. (Full story)However, some intelligence officials have downplayed the New York information, saying it was not credible.The threat, which was revealed by New York Mayor Michael Bloomberg three days ago, mentioned Friday and Sunday as possible days that an alleged attack might occur, according to one official with knowledge of the investigation.Friday was three months to the day that four bombers carried out attacks on three London subways and a double-decker bus, killing 52 people and wounding 700. The July 7 morning rush-hour attacks were the city's bloodiest since World War II.Kelly said that since city officials learned of the plot, security in New York has been stepped up. About 4.5 million people ride the subway each day. "We've increased our officers in the transit system," Kelly told "Fox New Sunday." "... We've increased our bag searches. You'll see more dogs in the subway system. "We've done quite a bit, and we're continuing to do that until we have a level of comfort that the threat has been reduced."Security heightened at baseball gameCounterterror police squads will be aboard the subway ahead of Sunday night's major league baseball playoff game.The New York Yankees and Los Angeles Angels originally had been scheduled to play Game 4 of the American League Division Series on Saturday afternoon, but the contest was postponed because of an all-day rain.More than 55,000 people are expected to attend the game, and many fans will ride the subway to Yankee Stadium in the Bronx.Police officials said the counterterror teams will also patrol during the game, scheduled for 7:55 p.m. ET.CNN's David Ensor, Deborah Feyerick and Jamie McIntyre contributed to this report. Copyright 2005 CNN. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed. Associated Press contributed to this report.