Lawmakers at government buildings in Ottawa, Canada have been warned not to open any unidentified mail after envelopes containing white powder were delivered to two senators’ offices Monday.

Canadian police are investigating the suspicious mail after noticing some senators had received the envelopes, which were also labeled, “Ottawa shooting,” Canada’s Globe and Mail newspaper reported.

The country’s Senate Protective Service sent out a note Monday afternoon alerting parliamentarians to the envelopes. Several more envelopes were found, a Senate spokeswoman said, but she did not elaborate on to whom they were sent.

“The envelopes and their contents have been secured,” the Senate Protective Service said in an email to Parliament Hill staff.  “Unless your mail is from a confirmed source – we ask that you do not open it,” the email said.

Newfoundland Senator David Wells said his via Twitter that his office received one of the envelopes in question.

It’s not clear whether the as yet unidentified powder is dangerous, the security service said. No employees who were in contact with the envelopes have displayed any signs of illness.  

Four letters containing white powder were sent to the Quebec riding offices of four federal cabinet ministers earlier this month. Provincial police later confirmed the packages posed no danger.