The initial arrests were made on Wednesday. Miami-Dade County police said they are also investigating American Airlines ticket agents and others who may have worked with the skycaps.
The skycaps worked for the company Eulen America, which provides janitorial, baggage and security support at Miami International and 11 other U.S. airports, as well as nine in Latin America. Both Eulen and American Airlines said they are working closely with authorities. The bags all went on American flights.
Transportation Security Administration Miami Security Director Mark Hatfield said he did not believe the scam posed a security risk.
"In any criminal investigation, we always look for a nexus to security. In this case, we're satisfied that there is none," he said.
But police spokesman Detective Roy Rutland said investigators think some of the luggage checked by the skycaps could have gone onto planes without passing through security.
"There were bags that may have been moved by baggage handlers - working with others - onto the aircraft without getting scanned," Rutland said.
All checked bags at the airport are supposed to be inspected by the TSA. Those checked at the curb are immediately put on a belt that goes directly into the TSA security area. In the American Airlines terminal where the scheme allegedly took place, those checked at the ticket counter are hand delivered by passengers or porters to the TSA inspection machines. All other luggage carried onto a plane must pass through the passenger security checkpoint area.
Skycaps at the Miami airport can waive fees for wheelchairs, strollers or for bags belonging to military personnel. Police say they used those exceptions to waive fees and check in unreported boxes and bags.
"We take these matters quite seriously and have no patience for this type of behavior. Those arrested are no longer employed by the contractor and are not performing any work on our behalf," American Airlines said in a statement.
Eulen CEO Luis Rodriguez said his company was "deeply saddened by the situation." But he added in a statement: "It does not reflect on the approximately 1,400 Eulen America currently employed at Miami International Airport, or the more than 31,000 employees working throughout the Americas."
Rutland said the investigation began in February when American Airlines alerted police to unclaimed and unregistered baggage that was ready to go to Bolivia without a passenger. Investigators believe the travelers were mostly from individuals shipping goods to Latin America and that the scam may have netted the skycaps tens of thousands of dollars. Those sending the baggage were likely seeking to avoid cargo fees by shipping merchandise as luggage, he said.
The extra baggage would have added weight to the plane, but Steve Lott, spokesman for the Air Transport Association, which represents the major airlines, said it was unlikely enough to have posed a safety risk. At most, the extra baggage would have caused the aircraft to use up more fuel, but airplanes always fly with a reserve, he said. A large increase in luggage would have been most significant on a smaller aircraft, but it would have been noticed.
"If you have a small plane with 30 people, and the crew loads on a significant number of bags, that's going to raise some flags," he said.