MUNICH — Munich Airport was forced to suspend flights for the second time in less than 24 hours late Friday after suspected drone sightings, leaving thousands of passengers stranded and intensifying European concerns over mysterious aerial incursions.

Operations were halted “as a precautionary measure due to unconfirmed sightings,” the airport said in a statement.

Federal and state police were tasked with investigating and defending against drones.

The shutdown followed Thursday night’s closure when 17 flights were grounded and 15 others diverted to airports in Germany and Austria.

Flights resumed Friday morning, but by evening, more sightings triggered a renewed suspension.

Police said helicopters and other assets were deployed but no drones were located.

Hundreds of passengers spent the night in terminals on cots or were transported to nearby hotels.

The incidents are part of a wave of unexplained drone overflights at airports and critical infrastructure across the European Union.

Belgian authorities confirmed drones were spotted overnight above a military training base near the German border.

German Interior Minister Alexander Dobrindt said he would meet European counterparts in Munich this weekend to coordinate a “drone detection and defense plan.”

“We are in a race between drone threat and drone defense. We want to and must win this race,” Dobrindt said.

Authorities have not identified who is responsible for the overflights. Some European officials suspect Russia, though experts say non-state actors could also be behind them.

The closures come amid heightened security concerns, with EU leaders pledging stronger measures against drone threats following a summit in Copenhagen this week. — Agencies