"Just FYI flight 2212 to Houston was delayed an hour because you let this guy on the plane wearing a gas mask," Joseph D S wrote on Twitter. "This then panicked people on the plane and we had to wait for him to be escorted off."
The man was wearing a dark-colored coat and a toboggan on top of the mask, which covered his entire face. It's unknown why he was wearing it or if he was charged with a crime.
Flight 2212 was scheduled to leave DFW International Airport just before 7 p.m. Thursday but took off 50 minutes late. It arrived at 9:07 p.m. at Bush Intercontinental Airport in Houston.
American Airlines representatives responded to the initial tweet.
"Our team is aware of this and worked with local officials," they wrote.