TEHRAN-German airline Lufthansa says its Friday flight from Frankfurt to Tehran is cancelled “as a precautionary measure” because of the security situation for airspace around the Iranian airport.

The same flight turned back Thursday for the same reason as investigators rush to determine what brought down a Ukrainian airliner this week near Tehran. All 176 people on board were killed.

“As soon as we have detailed information, we will decide if and when our Iranian flights can be operated again,” Lufthansa said in a statement.

Some Western leaders say the plane appeared to have been unintentionally hit by a surface-to-air missile amid tensions between the U.S. and Iran. Iranian officials have ruled out a missile attack. The French air accident investigation authority says it has been invited by Iran to join the probe into this week’s plane crash near Tehran.

The French authority known by its French acronym BEA said Friday it has been notified by Iran’s air accident board. The BEA in a Twitter post says it has designated an accredited representative to the investigation.

The plane’s engine was designed by CFM International, a joint company between French group Safran and US group GE Aviation.

French Foreign Minister Jean-Yves Le Drian earlier Friday told RTL radio that “it is necessary to establish the truth” and called for “total clarity.” He wouldn’t comment on whether the plane could have been hit by an Iranian missile, which the U.S. has asserted.