On a flight from Seattle to San Jose, Calif., on Monday, a woman caught a glimpse of some text messages that alarmed her.

A man seated in front of her on the Southwest Airlines flight was “texting about sexually molesting young children,” the San Jose Police Department said in a news release.

The man, identified by police as Michael Kellar, 56, of Tacoma, Wash., was using a large smartphone with enlarged text, making it easier for her to read his messages and take photographs of them.

The woman alerted a member of the flight crew, who notified police officers working in the Mineta San Jose International Airport. Mr. Kellar was arrested after the plane landed.