Story highlights Two Boeing 757s nearly collided off Hawaii on April 25

United Airlines Flight 1205 had to descend rapidly to avoid a collision

A passenger on the United flight wrote a blog about the incident

Commercial airline pilot says the collision avoidance systems worked as designed

Two commercial jets that nearly collided north of Hawaii last month were at altitudes assigned to them by air traffic controllers, the National Transportation Safety Board tells CNN.

The cause of the near-disaster in the skies east of the Hawaiian Islands on April 25 is still under investigation. But a source with knowledge of the probe said the early focus is finding out why the planes were assigned to altitudes that put them dangerously close.

The incident was brought to public attention when a passenger on one of the planes wrote a blog titled: "Two Weeks Ago, I Almost Died in the Deadliest Plane Crash Ever."

In the incident, United Airlines Flight 1205, which was headed east to Los Angeles from Kona International Aiport on the Big Island of Hawaii, had to descend quickly to avoid a westbound US Airways jet, approximately 200 miles northeast of Kona.

The United pilots received an audible warning that the plane was in danger from its traffic collision avoidance system, which monitors airspace around a plane. Both planes were Boeing 757s.

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Flight-tracking websites show the United plane descended 600 feet in 60 seconds.

The passenger who wrote the blog post, Kevin Townsend, described feeling weightless as the United plane suddenly plunged.

"I felt my body float upwards and strain against my seatbelt," Townsend wrote. "Passengers around me screamed. There was a loud crash in the back -- a coffeepot clattering to the floor and tumbling down the aisle. Our tray tables began rattling in unison as the 757 strained through the kind of maneuver meant more for a fighter jet."

Townsend estimated a total of 590 people were on the two planes -- based on the assumption that the other plane carried the same number of passengers and crew as the 295 people aboard his flight. Thus, according to Townsend, the death toll in a collision would have been the worst air disaster in history, surpassing the 583 who died in the 1977 runway collision of two Boeing 747s on a runway on the Spanish island of Tenerife.

However, airlines officials this week declined to say how many people were on the two passenger jets in last month's incident.

Townsend said he believes it was his report that led the Federal Aviation Administration to investigate. He said he wants to make sure near collisions are thoroughly investigated.

"I don't think there's some epidemic of near accidents that occurring, but it was a jarring experience dodging another plane," he said.

An FAA official, speaking on background, said the agency began investigating immediately after the incident, and independent of Townsend's report.

On Thursday, a joint FAA-NTSB team arrived at the Honolulu Control Facility to continue the investigation, the NTSB said.

Commercial airline pilot Les Abend said the collision avoidance systems worked as designed.

"You're going to run into this situation, unfortunately. Nothing is perfect. I know that's not a comforting answer, but indeed the system worked," Abend told CNN.