Washington (CNN) -- A United Airlines Boeing 777 jetliner came dangerously close to a small Cessna aircraft over San Francisco, California, on Saturday, according to federal officials who launched an investigation into the incident Tuesday.
The planes came so close that a collision avoidance system sounded, and pilots of the larger aircraft later reported they could see only the underside of the Cessna 182, a small, single-engine aircraft.
The National Transportation Safety Board launched an investigation Tuesday and said it was too early to suggest who or what was at fault.
But an FAA spokesman pointed at possible controller error, saying the controller should have noticed the Cessna earlier.
According to the FAA account, as the United flight departed San Francisco International Airport, the controller scanned a radarscope and spotted the Cessna flying southbound at 1,500 feet about two miles west of the airport.
The controller alerted both pilots to the proximity of the other aircraft, the FAA said. The Cessna pilot reported that he had the Boeing 777 in sight, and adjusted his path to maneuver above and behind the jetliner. The United pilot received an audible alert "and leveled their climb in response to that alert," the FAA said.
The FAA said the planes were 600 feet apart vertically and 3,000 feet laterally when the controller first contacted the pilots, and never came closer than 300 feet vertically and 1,500 feet horizontally.
In the airspace surrounding airports, controllers typically maintain at least 500 feet vertical separation and 1.5 miles horizontal separation, an FAA official said.
The NTSB said it learned of the incident because of a three-week-old rule that requires airlines to report to the NTSB any time a plane takes action to avoid a collision because of an alert.
In this circumstance, United Airlines Flight 889 had been cleared for take-off from San Francisco International Airport about 11:15 a.m. Saturday for a flight to Beijing, China. It was carrying 251 passengers and a crew of 17.
The first officer, who was flying the aircraft, reported the plane was at an altitude of about 1,100 feet when an air traffic controller reported a plane at his "1 o'clock" position, the NTSB said. Immediately following the controller's warning, the planes traffic collision avoidance system -- or TCAS -- issued an audible warning.
The pilots saw the Cessna plane in a hard left turn traveling from their 1 o'clock to 3 o'clock position.
The first officer of the United plane pushed the control column forward to level the jetliner, and both pilots reported seeing the belly of the Cessna as it passed within 200 to 300 feet of them.
The NTSB said it is sending investigator Scott Dunham to San Francisco to begin the investigation.
United Airlines spokeswoman Robin Urbanski said the airline is cooperating with the investigation.
The owner of the Cessna could not be reached for comment.