National security officials have been notified that an underground tunnel was discovered near a venue for this summer's Pan Am Games in Toronto, CBC News has learned.

WATCH LIVE | Police set to hold news conference about tunnel at 10 a.m. ET

The tunnel was dug in the woods near Toronto's Rexall Centre and York University's Keele campus. It was large enough for a person to stand in, at around 2½ metres in height, and was about seven metres long. The tunnel had lights inside, powered by a generator, and the walls and ceiling were reinforced.

The reinforced tunnel was found near York University and the Rexall Centre, a venue for the Pan Am Games. (CBC)

Sources told CBC News that the tunnel was filled in by authorities.

A Toronto and Region Conservation Authority employee reportedly found the tunnel while walking near the TRCA headquarters at 5 Shoreham Dr. in mid-January. The employee, who was carrying out day-to-day activities, noticed a piece of corrugated steel in the woods, and when it was lifted, the tunnel could be seen about three metres down.

The TRCA told CBC News on Monday that it is aware of a police investigation.

Authorities have essentially ruled out the possibility that the tunnel was going to be used as a drug lab or marijuana grow-op, according to sources. It likely took weeks if not longer to dig, and the earth excavated from the tunnel appears to have been removed from the site to help avoid detection.

An illustration depicts the tunnel, which has been brought to the attention of national security authorities. (CBC)

Toronto police have refused to comment on the mystery tunnel, but are expected to hold a press conference about it Tuesday.

Tennis Canada, which operates in the Rexall Centre, is co-operating with the police investigation, a spokesperson said.

The Rexall Centre, Canada's national tennis facility, is set to be a tennis venue for this summer's Pan Am Games and the Parapan Am Games.

CBC News has learned that investigators visited the Tennis Canada site a few weeks ago and spoke with grounds crew at that time. While investigators were on site, there was also a police presence in the woods nearby.

The tunnel, marked above with a red circle, is adjacent to the Rexall Centre, Canada's national tennis facility. (CBC)

The former assistant director of intelligence at the Canadian Security Intelligence Service said the discovery raises some important questions.

"First and foremost is the question around the context of the tunnel — where it is positioned? How deep was it? How accessible was it? And what sort of things could be sent through that tunnel, being people or material," Ray Boisvert said.

A hole in this fence, which has since been fixed, led to the mystery tunnel. (John Lancaster/CBC)

Boisvert said that the tunnel's proximity to the Rexall Centre could be a concern for those involved in security for the Games.

"I would think that they would want to have some level of assurance that this was not targeting the Games or targeting any other facility around there," he said.

"I would want to be able to have some sense that we know what the purpose of that particular tunnel was and who was likely — if not who, precisely — was behind it."

The Ontario Provincial Police, which is leading the Integrated Security Unit for the games, told CBC News that it was unable to comment on the tunnel, as it was "a matter for York University and the Toronto Police Service."

On Monday, the university referred questions about the tunnel to the Toronto police.

No one has come forward to claim the digging tools and generator found at the tunnel or offer any explanation for its existence.

CBC News has confirmed employees at several area gas stations were asked if anyone had been filling up gas cans recently, in order to refuel the generator.

So far, no suspects have been identified.

If you have any information on this, or any other story you would like investigated, please contact CBC Toronto’s investigative unit: sarah.bridge@cbc.ca and john.lancaster@cbc.ca