An accident triggered by a fire and explosion in a subway tunnel along Metro’s busy Red Line near Union Station killed one man and seriously injured two transit agency workers, disrupting elaborate maintenance and rebuilding plans on the system.

In a carefully orchestrated shutdown that started at 10 p.m. Friday, Metro had turned off the power on the tracks, closed a busy Red Line stretch from Farragut North to Union Station and brought in about 100 Metro workers and contractors to do maintenance with a dozen different pieces of equipment on the tracks.

But early Sunday morning, things went drastically wrong.

About 400 feet from the Union Station platform, in the direction of Glenmont, flames erupted and a “loud noise” boomed in the underground tunnel near a piece of equipment on the tracks, according to Metro. It was reported as an explosion by some people on the tracks.

Initially, Metro officials said they thought that leaking hydraulic fluid from an underground vehicle apparently was ignited by welding equipment, causing the explosion and fire. That somehow caused a long and heavy piece of iron rail to move and strike the three workers, who were about 80 feet away. They were part of a crew that was ripping out old sections of rail along the closed section of track and putting in new ones.

One Metro worker who was on the mezzanine level of the then-closed Union Station when the incident happened said another worker came toward him.

“He was running, and he said: “ ‘Hey, everybody, get out. This thing might blow up in the tunnel,’ ” the worker recalled Sunday. He spoke on the condition of anonymity because he was not authorized to discuss the accident.

Workers in the tunnel put out the fire with an extinguisher before D.C. fire personnel arrived, according to workers on the scene and Metro officials.

Metro said it “is not yet known what caused” the 40-foot-long piece of rail to fall.

Officials identified the contractor who was killed as Harold Ingram, 41, of Virginia. He died “as a result of being struck by the piece of rail,” according to Metro.

Ingram worked for the Holland Co., a Crete, Ill., firm that is under contract to do welding services for Metro. The identities of the two injured Metro workers were not released. One is a supervisor, and the other is a track worker. They suffered serious injuries that were not life-threatening and are being treated at hospitals, according to Metro.

No passengers were injured.

At one point Sunday, Metro considered opening the closed-off portion of the rail system to passengers. After the maintenance shutdown had begun late Friday, passengers had been boarding free shuttle buses at Union Station and Dupont Circle, which took them through downtown and around the closed-off tracks, before getting back on the rail line to their destination. Nearly a quarter of Metro’s riders who use the rail system on weekends were affected by the closings.

But Metro officials ultimately decided against reopening the Red Line early, focusing instead on putting the rail back as crews also worked underground to get equipment off the tracks and prepare for a 5 a.m. opening Monday.

Metro is investigating the accident and has said that it has notified the Tri-State Oversight Committee, a watchdog group, and the National Transportation Safety Board about the incident.

“Our first order of business after attending to the injured is to prevent anyone else from being hurt,” said Metro Chief Safety Officer James Dougherty. “We also need to ensure we have the safest possible working conditions going forward, which means learning lessons from the investigation of this accident.”

In a statement posted online Sunday, the NTSB said it had been informed of the incident at Metro but would not be sending investigators to the scene because of the staff furloughs.

NTSB said it could only “engage in those activities necessary to address imminent threats to the safety of human life or for the protection of property.”

The Metro accident did not meet those criteria for exempting staff members from furloughs, according to the NTSB, but it asked Metro for its investigation report.

NTSB Chairman Deborah A.P. Hersman said in an e-mail that because the public affairs staff was furloughed, the agency could not provide additional updates to the news media.

Officials at Metro said Sunday afternoon that the root of the fire and explosion “has not yet been determined” and that it “is not yet known if there was a fluid leak or another mechanical issue.”

Jordan Wolf, president of the Holland Co., said Sunday afternoon that his company’s safety and operations staff were on the scene to “conduct a thorough investigation and find out exactly what happened.”

Officials at Metro said they were inspecting all “contractor-owned and [Metro] equipment at the site.”

Sunday’s fatal accident took place in an intense, busy construction zone where workers in hard hats, jeans and steel-toed boots performed tasks amid constant noise from diesel-smelling generators and machines that grind down parts of the rail to get them in place and other pieces of equipment continuously buzzing.

Metro officials pride themselves on putting the transit agency’s employees and contractors through regular safety training sessions known as a “Roadway Worker Protection” program that was designed after a series of incidents where employees were injured and killed on the rail system.

The accident Sunday marred work over the weekend that Metro officials had planned for six months as part of the transit agency’s $5 billion capital effort — part of its largest reconstruction since it was built 30 years ago.

Parts of Metro’s five rail lines have been shut down nearly every weekend for the past two years as part of an aggressive strategy by General Manager Richard Sarles to rebuild what he says is a system that has not been well maintained. The nearly back-to-back weekend closures on Metro are expected to continue until 2017 on the five lines. For crews, the shutdowns have become almost routine.

Metro has long said the weekend shutdowns provide more continuous time to get workers and slow-moving, heavy equipment onto the tracks so they can work more efficiently — and safely — without worrying about trains passing through the area with riders. In this weekend’s shutdown, crews had 55 hours to work on a half-dozen projects, which included installing cables for emergency power boxes, taking out old fasteners, bolts and rail, and putting in new parts. While the Red Line was closed downtown, riders could use the Blue and Orange lines at Metro Center and the Yellow and Green lines at Gallery Place.

Until the accident, Metro’s work on the weekend shutdown had gone without any problems. Some riders were surprised Friday night and Saturday to find the closure of the key stations. Others complained that it took longer to get to their destinations.

The weekend work comes as Metro has been losing ridership. This past week, Metro was especially hard hit because its ridership dropped about 20 percent because of the federal government’s furlough.

Metro’s employee safety record had been under particularly close scrutiny a few years ago when seven workers were killed in a series of incidents between 2005 and 2010. In an incident in January 2010, two Metro track workers were struck and killed by a piece of rail equipment near the Rockville station.

On Sunday, Metro’s top board members said they expect a full investigation from the agency’s safety department. Board Chairman Tom Downs said there were no concerns about the safety of the rail system.

“This was a construction accident,” he said. “It was not an operating safety issue.”

Mortimer L. Downey, a Metro board member who chairs the safety committee, said the weekend shutdowns are “extraordinary events” in how much coordination and planning they involve.

“They are what we need to do to get the system back in a state of good repair, and certainly we want to do it in a totally safe fashion,” he said, adding that he was “very concerned that someone lost their life as part of this.”

“If there are things that we should be doing we will do them,” he said.

Mark Berman, Martin Weil and Gazette reporter St. John Barned-Smith contributed to this report.