Transport Canada is investigating a strange incident involving a black helicopter at a prison in northern Nova Scotia last week.

Staff at the Springhill Institution reported seeing a helicopter hovering over the prison parking lot last Thursday. The observer couldn't see any markings, but estimated it was within 160 metres of the ground.

Transport Canada checked a radar playback of the time in question. It found a target in the vicinity operating about 400 metres above the ground.

That target originated from the Trenton area and returned there before dropping off the radar.

"Transport Canada is investigating an alleged December 4 incident, in which a helicopter was reported hovering over the parking lot of the Springhill Institution," Steve Bone, a spokesman for Transport Canada, said in an emailed statement.

"As the incident is under investigation, no further information is available at this time."

Helicopters involved in jailbreaks

In June, three inmates escaped from a jail near Quebec City when a helicopter swooped into the jail yard and picked them up. They were recaptured a couple of weeks later.

Last year, two inmates at another Quebec jail escaped by climbing a rope hanging from a helicopter that was hovering overhead. The two men were located about 50 kilometres from the jail, later that same day.

In response to this summer's incident, no-fly zones were established over three Quebec jails.