Operations are returning to normal after air traffic was shut down for more than 4 hours on Friday following a maintenance upgrade that crippled communications.

When asked, the transportation secretary could not guarantee on Sunday that a similar air traffic meltdown would not occur again.

The Singaporean company’s disclosure added to the timeline involving Ms. Huang, a former executive who had a relationship with Jensen Huang of Nvidia. U.S. officials say she was spying for China.