HOUSTON >>Federal officials are investigating an incident in which two United Airlines planes may have flown too close to each other over Houston.

The close call happened after both planes took off from Bush Intercontinental Airport on the night of May 9.

The two jets came within less than a mile of each other and 400 feet apart in height, Federal Aviation Administration FAA spokesman Lynn Lunsford said Friday. Normally planes would be separated by at least three miles in distance and 1,000 feet in altitude.

An air traffic controller told the pilot of United Flight 601 to turn right after takeoff, then realized that the plane was heading into airspace normally reserved for planes on another runway. United Flight 437 had just taken off from that second runway, and the controller gave both pilots instructions to keep the planes apart, Lunsford said.

Lunsford said the FAA has taken steps to prevent similar events in the future.

Jennifer Dohm, a spokeswoman for United Continental Holdings Inc., said both United planes “were operating in compliance with their air-traffic clearances.” She declined to comment further.

According to the FAA, both planes were Airbus A320 jets. One had 120 passengers and a crew of five, while the other had 95 passengers and five crew members.

An even closer call occurred last month at Newark Liberty International Airport in New Jersey, prompting the FAA to change procedures for takeoffs and landings at the airport near New York City.

In that incident, a United Airlines jet that was landing and a smaller United Express jet taking off in the opposite direction came within about 200 feet in distance and 400 feet vertically of each other. The pilot of the smaller plane said that the United jet came “real close.”