United Airlines is keeping the grounded Boeing 787 off its schedule through June 5.

Investigators are still trying to figure out what caused a battery fire and another battery problem last month.

United spokeswoman Christen David says the plane could still fly earlier than that if a fix is found. At that point, it would be used as needed around United's system.

United says flights from Denver to Tokyo's Narita airport that were supposed to start on March 31 will be postponed at least until May 12. If the 787 isn't ready to go by May 12, United says it will begin the Denver-to-Tokyo flights once the 787 is ready.

The world's fleet of 50 787s has been grounded since Jan. 16