Officials with the Canadian Security Intelligence Service are investigating the theft of some top-secret documents.

The papers were stolen last month from the CSIS officer's car, while the officer was at a Maple Leafs hockey game in Toronto.

Some experts who are familiar with the case, say it's the most serious internal security breach since CSIS was established in 1984.

The Toronto police and CSIS have been searching for the documents, but so far, they've turned up nothing.

Investigators think it was just the work of petty thieves.