ASPEN, Colo. (AP) — Authorities on Tuesday were investigating the vandalism of Aspen’s natural gas system that left thousands of residents and visitors without gas in the middle of the busy holiday tourist season.

Investigators hope graffiti left behind might lead to those responsible.

Gas valves were turned off at three locations in the system that supplies natural gas to the ski resort town on Saturday night, Aspen Assistant Police Chief Bill Linn said. At two locations, “Earth First!” was found written on pipes, he said.

The environmental group has not claimed or denied responsibility for the outage, Linn said. Investigators have been in contact with the group, which indicated that it typically takes credit for actions it performs, he said.

A representative for Earth First! could not be reached for comment Tuesday evening.

The FBI has joined the investigation, which has involved knocking on doors to find possible witnesses and looking for surveillance cameras that might show those responsible.

“We are still plugging away at the investigation,” he said.

Police released a photo of one of the pipes marked with “Earth First!” in the hope that it might lead someone to recognize the handwriting or a previous use of the term.

Gas service was almost immediately turned back on after the tampering with the lines was detected by Black Hills Energy, the local utility, but some customers later reported not getting gas, Linn said.

On Sunday the utility announced that it had identified additional damage that required it to turn off service to about 3,500 properties to allow it to make repairs and re-pressurize the system, leaving most homes and businesses, including restaurants and hotels, without gas for heating or cooking for two nights.

Many people used electric space heaters to keep warm and the utility provided 4,000 heaters that were distributed by police on Monday.

After re-pressurizing the system, utility crews began going door-to-door to relight gas appliances, which they advised residents and businesses not to do themselves.

ASPEN — Authorities on Tuesday continued to investigate the vandalism of Aspen’s natural gas system which left thousands of residents and visitors without gas in the middle of the busy holiday season.

The FBI and state law enforcement officials were working with police to investigate the disruption, which began Saturday, Aspen assistant police chief Bill Linn said, The Aspen Times reported.

“They would have had to have some familiarity with the system” to pull off the sabotage, he said of those responsible. “They tampered with flow lines. They turned off gas lines.”

“Earth First!” was found written on a pipe at a natural gas pumping station near the ski resort town. That is the name of an environmental group but it has not claimed responsibility for the outage, Linn said.

The outage left more than 3,500 homes and businesses, including hotels and restaurants, without natural gas for heating or cooking, the area’s utility, Black Hills Energy, said. Many people used electric space heaters to keep warm and the utility provided 4,000 space heaters that were distributed by police Monday.

In order to restore service, crews must turn off the gas meters before going door-to-door to relight gas appliances, the utility said.