Washington (CNN) -- An aborted landing by a plane carrying Michelle Obama because it was too close to a military plane ahead of it was a "serious incident," Transportation Secretary Ray LaHood said Wednesday.
LaHood confirmed that the Federal Aviation Administration was investigating the incident Monday at Andrews Air Force Base.
The National Transportation Safety Board, an independent federal agency, also said it is launching a probe into Monday's incident, according to NTSB spokesman Peter Knudson.
"We consider any incident like this a serious incident," LaHood said. "It's under investigation, we will get to the bottom of what happened and how it happened, and obviously make sure it doesn't happen again."
He emphasized that the first lady was never in any danger and said he had yet to hear from either Michelle Obama or the president about it.
The planes -- which were both trying to land -- were three miles apart, when they are supposed to be five miles apart, a senior administration official told CNN, adding that it was believed to be an air traffic controller mistake.
Asked why the separation rules governing plane landings were not followed, LaHood said: "That's what we'll figure out in the investigation."
The FAA said in a statement controllers at the Air Force base instructed an incoming Boeing 737 to perform a "go around" "because the plane did not have the required amount of separation" behind the military plane.
"The aircraft were never in any danger," the agency said.
The landing was briefly aborted and Obama's plane had to circle, according to the senior administration official.
Obama was actually on a C-40, a military version of the 737 that was part of the Air National Guard -- not the regular Air Force fleet used by VIPs at Andrews, said Maj. Michelle Lai of the 89th Airlift Wing at Andrews Air Force Base.
The FAA did not want Obama's plane to be caught in the "jet wash" of the C-17 as it landed, Lai said, referring to the force of the air from the back of the C-17.
"It's important to know the FAA made the right call and at no time was the first lady's life in danger," Lai said.
When the Potomac TRACON, the regional radar facility, handed off the plane to the Andrews Air Force Base tower, the planes were three miles apart, a government official told CNN.
"Both facilities knew how far apart they were" at the time of the handoff, the official said. But the official declined to say why the hand-off occurred.
The TRACON could have slowed Obama's plane down or order it to turn earlier, the official said. Why that wasn't done is under investigation, but "it was a controlled situation," the government official said.
CNN's Ed Henry, Jeanne Meserve and Mike Ahlers contributed to this report.