A number of armed US air marshals have been grounded after high level security checks revealed they may have lied on their job applications.
The air marshals fly undercover, sometimes at short notice
The Transportation Security Administration said a "small percentage" of the undercover agents - authorized to use deadly force to thwart terrorist incidents - had been placed on administrative leave.
High level security checks on the background of the agents revealed "discrepancies" with what they told the TSA when they were hired.
The news follows the TSA's recent problems completing background checks for some of the 55,000 people employed to screen baggage at airports.
TSA spokesman Brian Turmail told BBC News Online the grounded agents may have lied about taking drugs in the past or failed to declare breaches of the law like under-age drinking.
"Anyone who flies as an air marshal will already have had extensive screening, like criminal records and finger prints checks and a thorough medical screening.
"These discrepancies were picked up during the final stage of background checks when we interview people who know the applicants," Mr Turmail added.
He rejected reports that as many as 100 marshals - of the several thousand believed to be operating - had been fired or put on leave, saying some people had quit their jobs and others had been put on leave pending the outcome of investigations.
Before the 11 September, 2001 terror attacks on New York, there were just 33 sky marshals, flying mainly on international flights.
Now the programme has greatly expanded, although the exact number of marshals is classified.
The programme expanded after the 11 September, 2001 attacks
They fly carefully chosen missions, sometimes on an hour's notice in response to new terrorist threats.
Earlier this month, TSA chief James Loy told Congress the agency fired 1,208 of its 55,000 baggage screeners - almost half for providing false information on applications, and others on revelations of drug or other felony convictions or other crimes.
The air marshal program has been criticised by some who say it suffers from high turnover and low morale.
Disgruntled marshals, legally barred from discussing the program, complained anonymously to members of Congress and reporters, saying hiring standards were lowered because of the haste with which the program expanded, Associated Press reported.