SAN FRANCISCO (AP) — Two airliners aborted landings at San Francisco International Airport last week after pilots spotted a Southwest Airlines jet taxiing down a runway where other planes were allowed to land.

An air traffic controller told the Southwest pilots that they should not have been on the runway during the May 19 incident.

The Federal Aviation Administration said Thursday that the Southwest plane had cleared the runway as other planes flew directly overhead, and the decision to abort the landing was “precautionary.”

“The FAA has investigated the events and determined that appropriate measures were taken to ensure safe operations,” the agency said.

The National Transportation Safety Board said it is not investigating the matter.

The incident is coming after half a dozen close calls in recent months which are being investigated by the security services. Including one in February when a FedEx plane flew about 100 feet (30 meters) over the top of a Southwest plane in Austin, Texas, following a dispatcher cleared both planes use the same runway.

In this month’s incident, a United Airlines plane flew just a few hundred feet (100 meters) above San Francisco Bay before the pilots saw a Southwest plane on the same runway and decided to abort the landing.

Shortly thereafter, the crew of an approaching Alaska Airlines plane saw the same Southwest plane crossing a second parallel runway, and the pilots also aborted the landing.

The United and Alaska planes circled and landed safely.

According to a recording made by LiveATC.com, an air traffic controller told the Southwest flight crew, “You should not be on the runway.” When one of the pilots tried to explain, the controller cut him off, saying, “I don’t need arguments.”

The incident was first reported by the San Francisco Chronicle. San Francisco Airport was the scene of a terrible near disaster in 2017when the pilots of an Air Canada plane mistook the taxiway for their runway and nearly landed on top of four other planes waiting to take off.

Despite the recent heated conversations, the acting head of the FAA said that the country’s air traffic system is safepointing to the absence of fatal crashes involving a US airline since 2009.

However, concerns about the close calls prompted the FAA to hold a “safety summit” in March. The agency said this week it would invest $100 million in improvements at 12 airports, but not San Francisco. reduce the number of “runway trips”, when an aircraft or airport vehicle is on the runway when it should not be.