American Airlines said it would inspect eight planes after loose seats were found on two different planes in the last three days.
American Airlines spokeswoman Andrea Huguely said Monday that there could be a problem with the way certain seats fit into tracking on the floor of the Boeing 757 planes.
On Saturday, a flight from Boston to Miami made an emergency landing in New York after three seats on the Boeing 757 came loose shortly after takeoff.
Loose seats were discovered Monday on a second plane that was headed from New York to Miami.
Some union leaders blame outsourcing.
"You lose control of your product," said Larry Pike, president of Transport Workers Union Local 567. "When you outsource it, you lose control of your product. You have no quality control. If it was done in-house, it would have been checked three times."
He said the same contractors who did work on the seats also do work on engines.
"It could have been very catastrophic," he said.
The airline acknowledges the seats that malfunctioned had been worked on by a contractor, but said the airline's own mechanics rechecked them when problems were detected.
"We had determined there was an issue with those seats," said American Airlines spokesman Bruce Hicks. "American maintenance had worked on them as well. In fact, they were the last to handle them, so this isn't American versus contractor. American has some of the best mechanics in the entire world, and they worked on these, too."
Hicks said the airline is still looking into how the seats became unsecured.
The Federal Aviation Administration also is investigating.