An Amtrak construction crew severed a fiber line in New Jersey, causing an outage that grounded flights at the three airports.

A construction crew working on an NJ Transit project severed a backup Verizon line, grounding flights for more than a day.

Damaged communications lines at a key air traffic control facility in Philadelphia had been repaired, allowing airports across the U.S. Northeast to resume operations after hours of disruptions, the nation’s transportation secretary said Monday evening.

The FAA issued ground stops at Northeast airports after an Amtrak crew accidentally cut a fiber line, causing widespread flight delays.