Two planes came dangerously close to colliding, but it wasn't known until a federal investigation was conducted. A United Airlines flight and an ExpressJet flight were only 400 feet apart when they passed by each other over Newark Liberty International Airport.

According to a National Transportation Safety Board report, a United flight from San Francisco and an ExpressJet flight heading to Memphis came close to colliding on April 24. The report says that the planes came within roughly 200 feet of each other laterally and were only separated by 400 feet vertically.

The United flight has 155 passengers and six crew members on board while the ExpressJet flight was carrying 47 passengers and three crew members, the Daily Mail reports.

The NTSB report claims that an air traffic controller waited for another plane to land on the east-west runway of the airport before clearing the ExpressJet to take off in the north direction. The United flight was roughly three miles away at that point. However when the ExpressJet flight started to take off, the United flight was only a mile away. When air traffic controllers realized how close the planes were, the United flight was ordered to abort its landing and ascend.

In a radio transcript, the ExpressJet pilot is head telling air traffic controllers that he was trying to keep the nose of the plane down as he was climbing altitude. He even told the tower that the United flight came "real close" to him during the incident.

The incident occurred near where two north-south runways intersect with Runway 29. Runway 29 is shorter than two of the other runways at the airport and is usually used to handle additional plane traffic and in the event of high winds which make the other runways too dangerous to use.

Neither plane was damaged and no one was injured in the close call. "We are working with the NTSB in its review of the incident," United spokeswoman Christen David told CNN. It can take a few months for a final report to be released.