NEW YORK (AP) — New York drivers apparently cheated on hundreds of tests to operate the most vulnerable vehicles on the road — from trucks hauling hazardous materials to school buses — and eight commercial drivers have been charged so far, state and federal officials said Thursday.

Nineteen suspects, including three security guards, have been arrested, officials announced Thursday. They were arraigned Wednesday and pleaded not guilty to charges linked to what officials say is an elaborate scheme to obtain commercial driver's licenses with purchased test replies.

Some drivers used pencils inscribed with coded answers to test questions, state Inspector General Catherine Leahy Scott said at a Manhattan news conference.

Scott said the arrests represent only the tip of the iceberg in a probe also involving the New York City Police Department and agents from the federal Department of Homeland Security. Investigators don't yet know exactly how many drivers might be on the road with criminally obtained licenses.

"This is just the beginning," the inspector general said.

Only one of those charged this week had gotten behind the wheel after the cheating was revealed to authorities by a person Scott called a "confidential source" with knowledge of the scheme but who was not part of it.

Jackson Gilles worked as a school bus driver for a bus company in Westbury on Long Island, according to the office of Manhattan District Attorney Cyrus Vance Jr., who worked with the inspector general's office, the U.S. Attorney's Office for the Eastern District of New York, and the Department of Motor Vehicles to shut down the cheating operation.

"Our investigation uncovered numerous people who paid others thousands of dollars for answers," Scott said. "They were potential drivers of the most dangerous vehicles on the road — tractor trailers, tow trucks, tanker trucks, double and triple trailers."

Officials displayed surveillance camera photos of security guards allegedly accepting cash bribes of up to $4,000 in the street near DMV offices in lower Manhattan and Harlem.

Federal immigration officials also are involved because some of the suspects are foreign-born and their immigration status is in question, Scott said.

Eleven of the suspects face mail fraud conspiracy charges, accused of organizing and operating the scheme; eight are charged with cheating on both written and audio tests.

Authorities said applicants would leave the test site and hand blank tests to one of the organizers of the scam or a "runner" who would take the form to someone who could fill in the correct answers to questions on subjects such as brake safety, loading hazardous materials and vehicle control. The completed test would then be returned to the test taker. In some instances, they received interim licenses immediately.

Some of the 19 suspects were freed on $150,000 bond; others remained in custody, Scott said.

The investigation was first reported Thursday by the New York Daily News.