Terror in the skies Tuesday over California: Five dangerous passes and at least two near-mid-air collisions, according to air traffic controllers.

One involved a UPS flight en route to Orange County, Calif., from Louisville, Ky. -- another a Boeing 757 (search ) passenger jet headed to San Diego from Detroit.

The incidents occurred shortly after controllers lost radio contact with 400 aircraft coming in and out of airports across the West -- when a computer unexpectedly shut down because technicians forgot to service the computer as required every 30 days. Because they did not dump the hard drive, the computer overloaded.

The backup system also failed, stopping radio transmissions to pilots in mid-sentence. Controllers describe the next 13 minutes as "chaos," as some planes began to converge.

So close was one incident, the onboard collision avoidance alarm sounded, forcing the UPS pilot to go into an abrupt climb to avoid a private jet below.

Late Wednesday, the Federal Aviation Administration (search) refused to call the two incidents near misses, saying the planes were at least 1,000 feet apart vertically and almost a mile horizontally. Spokesman Greg Martin called the chances of Tuesday's mishap "one in several million."

Martin said there was never a danger of a mid-air crash. The agency said it was looking into five instances in which planes apparently got too close to each other during the communications blackout.

In a statement, the FAA also said radio contact failed but radar coverage remained fully operational and aircraft were safely handed off to other air traffic control facilities.

The center hit by the blackout controls airspace for a vast region that encompasses California, Arizona, Nevada and parts of Utah. The shutdown caused a ripple effect throughout the country as planes bound for the Los Angeles region were held on the ground for about three hours.

"A required 30-day maintenance check on the primary radio and voice communications system was not performed," the agency said. "This system turns off if this check is not performed."

It said a backup system also failed because it "was not configured properly to ensure its availability in the event of the primary system's failure."

The problems could have been avoided if strict FAA procedures had been followed, the agency said.

Three workers filed injury claims, saying they were traumatized by seeing flights veer toward one another on radar without being able to do anything, Hamid Ghaffari, local president of the National Air Traffic Controllers Association (search), said.

Airport operations were back to normal Wednesday following the radio failure at an FAA center at Palmdale, north of Los Angeles.

During the outage, air traffic controllers could monitor planes on radar but were unable to communicate with them. Pilots had to switch to another radio frequency to communicate with other control centers that took over flights in the region.

"We couldn't do anything," Ghaffari said. "We can't do our job unless there is communication. If there are no communications, you are helpless."

Under FAA rules, planes must remain at least five miles apart horizontally and 2,000 feet vertically. In at least five cases, that safety bubble was violated, and in two cases planes came within about two miles of each other, Ghaffari said.

On-board safety equipment that includes a collision-avoidance system helped avert disaster, Ghaffari said.

"That was the hero of the night," he said.

FOX News' William Lajeunesse and The Associated Press contributed to this report.