Forty pilots were arrested after an investigation found they were receiving Social Security disability payments for various illnesses but were still licensed to fly, federal officials said.

The pilots claimed to be medically fit to fly airplanes. However, they may have been flying with debilitating illnesses that should have kept them grounded, such as schizophrenia, bipolar disorder, drug and alcohol addiction and heart conditions, said Marlon Cobar, a prosecutor with the U.S. attorney's office in Fresno.

FAA spokesman Donn Walker said he did not know how many flew for a living, but at least a dozen of them had commercial or airline transport licenses.

A review of 40,000 pilots in Northern California began in July 2003 as an investigation into the fraudulent use of Social Security numbers.

When dozens of names turned up in Social Security Administration and Federal Aviation Administration records, investigators "realized there was probably criminal wrongdoing -- either lying to the FAA or wrongfully receiving benefits," Cobar said.

The FAA revoked 14 pilots' licenses and medical certificates, which are necessary to maintain valid licenses, the U.S. attorney's office said. Others were referred for administrative revocation.

"You can't really fly a plane if you're telling the Social Security Administration you have a disabling back condition or bipolar disorder," Cobar said Monday.

Other pilots, who were not charged, were found to be lying about having illnesses to collect Social Security payments, Cobar said.

Thirty pilots were charged with making false statements to a government agency, and 10 are charged with making and delivering a false official writing.