WASHINGTON (CNN) -- Information that led to heightened security for the New York City transit system was a hoax, government sources said Tuesday.

The sources said an informant in Iraq who provided the tip had told investigators there was a terrorist plot involving New York's subway system. That informant admitted he gave false information, the sources said.

On Monday, New York police said they would reduce the increased security measures put in place last week on the city's subways.

Law enforcement sources said last week the person who passed along the New York tip also gave information that prompted a military operation in Iraq, which led to the arrests of three al Qaeda suspects in Musayyib, about 45 miles south of Baghdad.

Government sources said the three men arrested in Iraq with suspected links to the possible plot had been interviewed and underwent lie detector tests showing they knew nothing about such a plan.

New York Mayor Michael Bloomberg warned Thursday that the city had received information from the FBI about a "specific threat," prompting the heightened subway security.

The threat mentioned Friday and Sunday as possible days that an attack might occur, according to one official with knowledge of the investigation.

Friday came three months to the day after four bombers carried out attacks on three London subways and a double-decker bus, killing 52 people and wounding 700. (Full story)

After Bloomberg's warning, some intelligence officials played down the New York information, saying it was not credible.

On Monday, Bloomberg said he stood by the decision to increase security.

"Over the immediate future, we'll slowly be winding down the enhanced security, but we, remember, stay at level orange in this city," the mayor said, referring to the color-coded advisory system denoting a "high" risk.

"We have since 9/11, and we're going to take every single threat that has any chance of being credible seriously and do exactly what we did," Bloomberg said.

Source: CNN