TTC Chair Adam Giambrone said he and other transit officials would spend Thursday figuring out the cost of a construction mishap which shut down a major part of the Yonge subway line for six hours during the evening rush home.

Giambrone told reporters that the construction worker who inadvertently drilled a hole through the road and into a subway tunnel between St. Clair and Summerhill stations on Wednesday worked for Enbridge and was not hired by the TTC to do any work.

The hole "compromised" the safety of the tunnel, according to TTC engineers who said they were afraid concrete slabs would fall on the tracks or onto a moving train.

The mishap forced thousands of frustrated commuters onto shuttle buses as the TTC halted service from Bloor to Eglinton stations during the height of rush hour.

The subway was back up and running on schedule last night at around 8:30 p.m.

Transit officials issued an apology to their commuters and assured them the subway is once again a safe mode of transportation.

"TTC crews have secured a 75-foot cut slab of concrete to ensure the subway can operate safely," the TTC said in a news release issued Wednesday evening.

However, that fix is only temporary, and more work will have to be done. It could take a few weeks.

CTV Toronto's Janice Golding said on many parts of the Yonge line, the subway tunnels can be as few as 60 centimetres below the surface.

However, Giambrone said the contractors should have known.

"They had a permit to do work. They obviously did not have a permit to cut our subway structure," he said.

As a result, someone has to take responsibility for the cost of repairs, he said.

"It shouldn't be the TTC because it wasn't the TTC's fault," Giambrone said.

The contractor working for Enbridge was Link-Line. It was doing emergency work relocating a natural gas pipeline at the city's request, said Debbie Bourydis of Enbridge Gas Distribution.

"There were permits and there were locates and to say who would be taking ownership of the cost is really difficult to say right now," she told CTV Toronto, adding the investigation was not complete.

Link-Line declined to comment.

Giambrone said the TTC is not in a position to pick up  the tab for this incident, and he wouldn't rule out a lawsuit.

The TTC announced Tuesday it would be hiking fares in response to a $100-million shortfall in its operating budget.

With a report from CTV Toronto's Janice Golding