At least 175 flights were canceled and many others were delayed Saturday at airports across Japan after the nation’s main air control system broke down, government officials said.

According to airline officials, more than 20,000 passengers were affected.

All aircraft for domestic and international flights were grounded for at least 30 minutes after the failure, which involved both the main computer system and its backup.

Officials said they were still investigating the cause of the failure, but denied any possibility of computer hacking or other crimes. They said the incident did not affect aviation safety.

The computer trouble occurred at around 7 a.m.

Many airports began allowing planes to depart half an hour later in 10-minute intervals by manually managing traffic, according to the office of the Land, Infrastructure and Transport Ministry at Tokyo’s Haneda airport.

Haneda, Narita and many other airports shortened the takeoff interval to five minutes at around 8:30 a.m. after the system was partially restored, the office said.

The system was fixed shortly before 11 a.m., but the disruption of flight schedules continued at some airports.

About 1,000 flights were delayed for 30 minutes or longer, the ministry said, with the longest delay being five hours and 50 minutes.

The troubled flight data-processing system at the ministry’s Tokyo Air Traffic Control Center in Tokorozawa, Saitama Prefecture, automatically transmits flight information to airports across Japan. The system manages flight plans.

The ministry said that early Saturday it partially replaced programs in the system that exchanges flight plans with the Defense Agency. The system went down immediately after it was turned on following the replacement.

A transport ministry official said it was too early to link the change to the failure.

The air traffic center was forced to take alternative measures, which included telephoning airports to give flight information and inputting flight data manually.

The system has a backup, but both systems went down at the same time, according to the ministry.

The Tokorozawa center manages 70 percent of the airspace which comes under Japanese jurisdiction, the ministry said.

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