Rogue co-pilot Andreas Lubitz may have spiked his captain’s coffee with a substance to force him to leave the cockpit for the toilet — allowing the Germanwings flier to lock the door and send the plane into a mountain in the French Alps.

Lubitz, 27, changed the autopilot setting in the Airbus A320 to just 100 feet in his mass murder-suicide plot that claimed the lives of all 150 people aboard.

German prosecutors believe he may have added the chemical to Capt. Patrick Sondheimer’s coffee to make sure he’d leave his seat, the Daily Mail reported.

Investigators are examining Lubitz’s computer to find clues about the possible spiking. He had used his computer to research suicide methods and cockpit door safety, officials have found.

Lubitz reportedly urged Sondheimer to use the toilet — reminding him that he hadn’t taken a toilet break in Barcelona, Spain, before taking off for Duesseldorf.

It has been reported that Lubitz had broken off his pilot training for several months in 2009 and upon restarting, informed the Lufthansa pilot training school he had overcome a period of severe depression.

Lufthansa, the parent company of Germanwings, has said he passed all medical and suitability tests upon restarting training.

Meanwhile, the German aviation authority Luftfahrtbundesamt, or LBA, said Thursday that correct procedures were followed when Lubitz was awarded his pilot’s license, Reuters reported.

The LBA said over the weekend that it had been unaware of Lubitz’s depression. Lufthansa said that under regulations in effect until 2013, it was not required to inform the LBA.

Lufthansa head Carsten Spohr met with LBA president Joerg Mendel this week.

“We came to the joint conclusion that the correct procedures for awarding a pilot’s license were followed,” the LBA told Reuters on Thursday.