UNICH (AP) -- Germany's Munich Airport reopened Saturday morning after authorities shut it down the night before for the second time in less than 24 hours after two additional drone sightings, officials said.

The closures are the latest after mysterious drone overflights in the airspace of European Union member countries.

The airport, one of Germany's largest, reopened gradually beginning at 7 a.m. (0500 GMT) Saturday. Planes typically begin taking off at 5 a.m.

Federal police said two drone sightings were confirmed shortly before 11 p.m. Friday near the airport's north and south runways, the agency said in a statement Saturday. The drones flew away before they could be identified.

Delays were expected to continue throughout Saturday, the airport said in a statement. At least 6,500 passengers were impacted by the overnight closure Friday into Saturday.

The previous closure, Thursday night into Friday, affected almost 3,000 passengers.

Authorities were not immediately able to provide any information about who was responsible for the overflights.

The incident was the latest in a series of incidents of mysterious drone sightings over airports as well as other critical infrastructure sites in several European Union member countries. Drones also were spotted overnight in Belgium above a military base.

A drone incident in Oslo, the capital of Norway, which is a NATO member but not part of the EU, also affected flights there late last month.

It wasn't immediately clear who has been behind the flyovers. European authorities have expressed concerns that they're being carried out by Russia, though some experts have noted that anybody with drones could be behind them. Russian authorities have rejected claims of involvement, including in recent drone incidents in Denmark.

Alexander Dobrindt, Germany's interior minister, on Saturday cautioned the public that not every drone is a threat.

Even if a drone flight is initiated by a foreign power, he said, it does not automatically mean people are in danger. The flyover might just be a provocation, he added.

Still, the minister said drones are the latest arms race and Germany, along with the rest of the EU, must face it. Dobrindt said he is planning a joint drone defense center to coordinate between Germany's federal government and the states.

Germany's Munich Airport reopens after second closure in less than 24 hours due to drones