A man on a flight from Detroit heading to Logan International Airport yesterday was allegedly overheard telling another passenger that he was a federal air marshal. He wasn’t. And, unfortunately for him, there was a real one on the plane, who detained him, authorities said today. The incident happened on Delta Flight 1922, which landed safely at 11:35 a.m., the Transportation Security Administration said. “The individual is in law enforcement custody and the matter is currently under investigation,’’ the TSA said in a statement. TSA spokeswoman Ann Davis said she couldn’t say why the man had identified himself as an air marshal. “I really can’t speak to his intent,’’ she said.