A rapid gas leak into a basement utility room at a Silver Spring apartment complex caused a swift buildup of fumes, leading to the explosion that killed seven people and reduced four floors of apartments to rubble, according to investigators and the complex’s management.

The leak “was fast,” said Clark Melillo, president of Kay Apartment Communities, the management company.

The Aug. 10 explosion, which crumbled walls and led to the collapse of floors above the basement, also tore open a gas line, feeding more fuel into the inferno and making it difficult to extinguish.

The exact site of the gas leak and the source of what ignited it have yet to be pinpointed, investigators said Friday.

The probe is being led by the pipeline division of the National Transportation Safety Board, which took over as lead investigator Thursday from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives and the Montgomery County Fire and Rescue Service.

Crews in Silver Spring, Md., continued to look for bodies on August 16 and clean up after a massive fire and explosion six days earlier at an apartment complex. At least six people are believed to have been killed. (Video: Dan Morse/The Washington Post; Photo: Bill O'Leary/The Washington Post)

The NTSB generally investigates only two or three such pipeline disasters a year — selecting incidents in which the board’s conclusions could lead to widespread improvements in equipment or engineering.

“We don’t want 20 different accidents like this one,” said Ravi Chhatre, a senior NTSB pipeline investigator.

He said the federal probe will involve testing equipment pulled from the rubble and hopes that the investigation will be complete within a year.

Like many gas-heated apartment buildings, the one that blew up — at 8701 Arliss St. — receives high-pressure natural gas through a two-inch service line into a utility room. From there, the gas passes through a “regulator,” which reduces the pressure and sends it into a bank of meters that feed it to individual apartments.

In the utility room, the regulators, the meters and the pipes serving that equipment are managed by Washington Gas, according to Washington Gas and Kay Apartment Communities. The pipes coming out of the meters and into the apartment units are managed by Kay.

Investigators have focused sharply on the utility room gas system, and while searching through rubble, they found the building’s regulator, its backup regulator, the piping infrastructure and the bank of meters and positioned them on a piece of plywood — creating a mock-up of how the equipment was lined up when it was on a wall, which had been blasted out, according to Chhatre and investigators who were at the scene for seven days.

The effort to locate the source of the leak also has been aided by information from tests Washington Gas conducted after the explosion and fire.

Investigators of an Aug. 10 fire in Silver Spring have focused their attention on a basement utility room at 8701 Arliss Street – like the one pictured here, from 8707 Arliss Street. This photo was taken within the last week, and shows a bank of gas meters similar to the setup in the utility room that blew up. The gas comes in from the left, passes through a regulator that decreases the pressure, and is distributed to the meters linked to specific apartments. (Kay Apartment Communities)

Washington Gas tested the service line leading into the utility room and found it sound, company spokesman Jim Monroe said earlier this week.

That has helped lead investigators to believe the leak was probably inside the utility room and in the area of the regulator, according to several officials with direct knowledge of the case.

Monroe declined to comment Friday.

Chhatre said it’s too early to pin any blame on the regulators in the utility room. He said he expects to soon learn their age. “They’re quite old,” he said.

The source of the ignition may never be definitively known, however one person familiar with the probe said investigators are looking at the role of a gas-fired hot- water heater in the utility room adjacent to the bank of meters. It provided hot water to 14 apartment units, so it could have been operating before midnight on Aug. 10, when the explosion erupted, according to Kay Apartment Communities.

“It’s highly suspect,” said a law enforcement officer with direct knowledge of the case, speaking on the condition of anonymity because he did not have permission to discuss an ongoing investigation.

Officials are looking at other possible ignition sources as well, including those in an adjacent room or even one floor above the explosion site.

At a news conference Friday, Daniel Board, an ATF agent, said investigators conducted more than 100 interviews with witnesses and watched hours of surveillance video. He said investigators found no evidence of a crime or unauthorized entry into the utility room.

Also Friday, Montgomery County police named three victims of the fire whose identities have been confirmed and said they do not expect to recover more remains from the building. The three named lived in Apartments 101 and 103 — units directly above the utility room.

The victims confirmed killed are Saul Paniagua, 65, Augusto Jimenez, 62 and Maria Castellon, 53.

Identification of other remains removed from the debris continues and awaits DNA results, among other testing.

Some residents have recounted seeing families dropping children from windows to save them and of fleeing down stairwells that were collapsing around them.

In an interview Friday, Melillo, the property management president, said there were staffers in two adjacent rooms to the utility room until 6 o’clock the night of the explosion. None smelled gas, he said. “There was no gas in there when they left,” he said.

He also confirmed what officials had said earlier Friday: that there was no gas detector in the utility room. He said that depending on the results of the NTSB probe, the company may install such devices in utility rooms.

“This is an in­cred­ibly rare event. I am very eager to get the results of the investigation,” he said.

The NTSB’s Railroad, Pipeline, and Hazardous Materials Investigations division has four staff members who specialize in pipeline accidents, according to the NTSB. Two of the four are senior investigators, the NTSB said.

Chhatre, the senior pipeline investigator leading the Silver Spring probe, has been with NTSB for more than 15 years. He has an undergraduate degree in metallurgical engineering and graduate degrees in that field and in materials science.

In an interview, Chhatre said the NTSB uses several criteria to select which pipeline accidents to investigate, including fatalities, serious injuries and/or significant property damage. The board also assesses whether an investigation of a specific incident could have broader impact and maximize safety improvements.

Given the safety focus, Chhatre said, incidents that appear most likely due to a single error from operator hold less interest for the focus of the investigation than would a scene that could prompt enhancements in equipment or in industry-wide operations or regulations.

A resident of 8701 had reported a smell of gas at the apartment building on July 25, which Montgomery County firefighters responded to, county officials have said.

At the Friday news conference, Alan Hinde — a division chief for Montgomery County Fire and Rescue -- said firefighters used detection meters to search for gas throughout the building and “found no positive hits.” The July 25 call was the most recent to the building about the smell of gas, fire officials said.