The National Transportation Safety Board has dropped the union representing rail workers as a participant in the probe into Sunday's Metro-North derailment that killed four people in New York City.

The move came after the union's chief publicly disclosed details about the engineer at the controls when the train barreled into a 30-m.p.h. curve at 82 m.p.h.

The federal investigative agency usually seeks technical expertise from a variety of industry, government, and union sources as it conducts examinations into the causes of a deadly disaster. As part of the process, the outside parties agree to maintain confidentiality during the investigation.

But Anthony Bottalico, general chairman of the union, the Association of Commuter Rail Employees, told reporters Tuesday that he believed the engineer of the Manhattan-bound commuter train had nodded off moments before the train entered a curve and veered off the tracks.

Hours after his news conference, the NTSB announced it had removed the union from the probe.

"While we value the technical expertise that groups like ACRE can provide during the course of an investigation, it is counterproductive when an organization breaches the party agreement and publicly interprets or comments on investigation information," NTSB Chairwoman Deborah A.P. Hersman said in a prepared statement. "Our rules exist to avoid the prospect of any party to an NTSB investigation offering its slant on the circumstances of the accident."

At the news conference, Bottalico said the train's engineer, William Rockefeller, had dozed off moments before the derailment at a curved portion of track.

"He basically nodded. . . . He had the equivalent of what we all have when we drive a car - that is, you sometimes have a momentary nod," Bottalico said. "How long that lasts, I can't answer that. Only Billy can.

Joseph Szabo, head of the Federal Railroad Administration, said in a letter Tuesday that his administration and the U.S. Transportation Department "have serious concerns" following Sunday's accident and three others that occurred in New York and Connecticut from May through July.

Though a federal team has been working closely with Metro-North Railroad and its parent agency, the Metropolitan Transportation Authority, Szabo said: "Immediate corrective action is imperative."

The MTA said in a statement that it would work with federal agencies to improve safety, and was "conducting a comprehensive probe of the safety culture throughout the MTA." Railroad employees were also getting expanded safety briefings.

This article contains information from the Associated Press.