FBI spokeswoman Judy Orihuela said Thursday that agents are looking for common threads between the two cases.
In the most recent incident Wednesday in Boston, a flight attendant found the words "bomb on board, Boston-Miami" scrawled on a bathroom cabinet. The aircraft was evacuated and luggage searched, though no bomb was found.
On Sept. 17, a flight attendant found a threatening note in a bathroom. The note used the word "bomb" and said explosives might be on board, the FBI said. The flight had taken off from Miami International Airport around 9:30 p.m. and had to return about 40 minutes later.
The plane was searched and bags were re-screened after 168 passengers and six crew members safely disembarked. Nothing turned up in the search, according to the Transportation Security Administration.
The passengers stayed overnight in Miami and left the next morning for Boston.
American Airlines did not immediately return a call seeking comment.