Toronto ? Canadian authorities arrested 19 Pakistanis on immigration charges after a seven-month investigation found they may have posed a threat to national security, officials said Friday.

The 19 men, aged 18 to 33, were arrested in a pre-dawn raid on Aug. 14 in Toronto, Royal Canadian Mounted Police spokeswoman Michelle Paradis said.

The intelligence arm of Canada’s immigration department drafted a document after the men were arrested that said most entered Canada as students from Pakistan’s Punjabi province and faked documents to keep their immigration status.

The document showed that some of the men allegedly took flying lessons and once tried to enter the grounds of a nuclear reactor. Airplane schematics were found in their apartments.

“There is a reasonable suspicion that these persons pose a threat to national security,” the document said.

The document said one of the men was enrolled in flying lessons that took him over the Pickering nuclear power plant, about 20 miles east of Toronto. One of his instructors reported that the one-year course was taking him more than three years to complete.

Two others were turned away from the gates of the plant after claiming they wanted to take a shortcut through the site to walk on the beach in April 2002.

The document also said an address used by one of the men was linked to the theft of a nuclear gauge, a device used in construction and containing the cesium-137. The highly radioactive material could be used to make a dirty bomb.