(Adds Biden quote)WASHINGTON, Jan 11 (Reuters) - President Joe Biden has ordered an investigation into an FAA system outage that grounded flights across the country Wednesday morning and said the cause of the failure was unknown. Asked if the outage was caused by a cyberattack, he said, "We don't know." "They don't know what the cause is," Biden said. We don't know what the cause of it is." U.S. flights were grounded or delayed on Wednesday as the FAA scrambled to fix a system outage, with passengers told to check with airlines for updates.