Natural gas pipeline investigators with the National Transportation Safety Board will assume the lead role in determining how an apartment building in Silver Spring blew up last week and killed seven people, officials said Wednesday.

The development suggests that there were problems in a service line going into the building.

The NTSB, best known for investigations of airplane and train crashes, has a pipeline division. For natural gas issues, NTSB is generally focused on large pipes that move gas in public areas, up to buildings and sometimes into buildings. The large lines are the parts of the system that carry natural gas up to meters where the pressure in lines is reducedfor use by residents and businesses, according to Keith Holloway, an NTSB spokesman.

As federal pipeline inspectors took on a more prominent role, families of some still-unaccounted-for residents maintained their vigil near the cratered building and also have given authorities DNA samples at a mobile lab to try to help identify remains recovered from the rubble.

Investigators appear to have ruled out that an explosion was intentionally set. But they cautioned that an exact cause hasn’t been determined and that they continue to analyze evidence they have been collecting over the past week.

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

The Aug. 10 explosion and fire shortly before midnight tore through 14 apartment units of a complex along Arliss Street, about 1.5 miles east of downtown Silver Spring. The natural gas lines on Arliss Street, and the service lines leading up to the building meters are managed by Washington Gas, according to a spokesman for the utility.

Washington Gas said Tuesday that it had tested its infrastructure in the area, and “determined it to be sound,” according to utility spokesman Jim Monroe.

Monroe said those tests evaluated the service line leading to the most heavily damaged apartment building, 8701 Arliss St. But because of the significant damage to the building, Washington Gas was not able to test the lines leading up to the meter and through the meter, Monroe said Wednesday.

Monroe declined to discuss what may have led to the explosion. “I think any speculation on cause, at this point, is premature,” he said. Monroe said Washington Gas has cooperated in the probe over the past week.

“We have and will continue to support the NTSB in every way we can,” Monroe added.

Since Thursday, NTSB pipeline investigators had been assisting at the Silver Spring scene, which has been led by investigators from the Montgomery County Fire and Rescue Service and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives.

Angelica Alvarez, left, and 3-year old Lesley Turcios watch as reporters talk to her neighbors in the Flower Branch apartment complex. (Bill O’Leary/The Washington Post)

“Based on some of the potential safety issues that have to come to light, and to possibly prevent it from happening again, we have launched an investigation,” Holloway said Wednesday. “All lines leading up to the meter — inside or outside — will be of interest to the NTSB.”

For seven days, rescue workers have combed through the debris. They have found remains of seven victims, but have yet to match the remains with seven unaccounted-for residents.

“The remains are in such conditions that visual inspections are not revealing the identity of these people,” Montgomery County Assistant Police Chief Russ Hamill said Wednesday.

Investigators are trying to use DNA and, if possible, fingerprints and dental records. Hamill hopes to start making positive identifications by Friday. “This was a horrific event,” he said.

He said police investigators will be at the scene at least through the weekend. “Investigators are hopeful that there will be no more victims found,” Hamill said.

Relatives of Augusto Jimenez Sr., 62 and Maria Castellon, 53, who lived in apartment 103 have been camping outside the destroyed building.

“Sometimes we think it’s a nightmare,” said Sandra Jimenez, 40, Augusto Jimenez’s youngest sister. “Then we see the wreckage and we have to accept that we are waiting for the bodies of our brother and our sister-in-law who was like a sister to us.”

News of the blast and fire was a hard blow, said Sandra Jimenez, who rushed to the building with other relatives as flames still burned and had hoped to find her brother and sister-in-law alive.

“After all these days, the most difficult thing is the wait. It is disheartening,” she said, speaking in Spanish. “It is hard to see people removing the rubble from my brother’s home.” The family “never imagined living this tragedy.”

One of the couple’s children, Augusto Jimenez, 35, gave authorities a DNA sample Friday in the mobile lab near the site of the blast. A week later, relatives say they are still waiting to receive remains of their loved ones for proper burials and haven’t been told when that may happen.

And the painful wait isn’t just in Silver Spring. In La Paz Centro in Nicaragua, the couple’s two other children and Jimenez’s 79-year-old mother also wait.

Jimenez and Castellon had built a home there and visited every year. They planned to leave Washington’s winters and retire there.

For now, Sandra Jimenez said, relatives and friends 3,500 miles apart are mourning.

“We are having the wake without the bodies,” she said. “We will take their ashes home and leave them in the home they worked so hard to build.”