The gasoline vapors that forced the evacuation of part of Franklin’s downtown Thursday originated from a leak in an underground fuel tank at a Central Street gas station, officials say.

“For an unknown amount of time, an unknown quantity of liquid gasoline was leaking into the ground … from a pipe that was running below one of the pumps at the Shop Express gas station,” said Michael Foss, Franklin’s fire chief.

The gasoline “found its way into void spaces under Central Street” and vaporized, he said.

“Vapors found their way into buildings, foundations, apartments, condos – that was the source of the smell,” he said.

The tanks at the store at 449 Central Street were being pumped out Friday morning and the leak has stopped. The New Hampshire Department of Environmental Services has taken over the investigation to determine if any gasoline, which is carcinogenic, has polluted waterways and to work on long-term mitigation.

“We believe the vapors are not a hazardous level but we still want individuals who smell gas to call 911,” said Foss.

Residents and businesses in the 300 and 400 block of Central Street reported the smell of gasoline Wednesday evening, leading to the evacuation of all buildings, including restaurants that were in the midst of dinner service. At least four people were put up for the night at the city’s community center.

“We ventilated buildings and the sewer system, were able to draw some vapors out of that system,” said Foss.

The evacuation order ended around 3 p.m. Thursday.