The New York Police Department, already on high alert after the ambush killings of two officers last month, issued a call for further precautions in the wake of a deadly attack in Paris, including telling its patrol force not to sit in pairs in squad cars when at a fixed post.

In an internal department memo, sent to all police precincts and commands by the chief of department, James P. O’Neill, patrol officers were reminded not to use cellphones except for official business and ordered to remain on guard when at a fixed post, the New York Times  reports.

“At least one officer must stand outside the vehicle at all times,” read the memo, circulated just after 7 p.m. Wednesday.

Officers were also told to pay close attention “to approaching vehicles and anyone getting out of a vehicle” and “to people as they approach and look for their hands as they approach you.”

The precautionary measures are on top of those already in place since Dec. 20, when Officers Rafael Ramos and Wenjian Liu were killed in their patrol car by a lone gunman targeting officers. They were disseminated two days after two other officers were shot and wounded in the Bronx.