The FBI on Wednesday was trying to determine whether a passenger staged a bomb hoax that prompted a search of a Thai Airways jetliner at Los Angeles International Airport. "The working theory at this point" is that a passenger aboard Flight 794 scrawled the bomb threat on a restroom mirror, but no arrest was made and the investigation continues, FBI spokeswoman Laura Eimiller said. There was no indication that a crew member was involved, she added. Making a phony threat against an airliner is a federal crime that carries a maximum sentence of five years in federal prison, Eimiller said. The plane, carrying 171 passengers and 18 crew members, was heading from Bangkok to Los Angeles when the threat was discovered. Thai Airways President Piyasvasti Amranand told The Associated Press that the message — written in English with bad grammar — warned that a bomb was on the plane. "This must have caused some nuisance to the passengers," he said by phone from Singapore. "It sounds like it was done by a crazy person." The Airbus A340-500 landed safely just before 9 p.m. Tuesday and was taken to a remote area of the airport, where FBI investigators interviewed the passengers, searched the plane and determined the threat was a hoax. "It was cleared last night by the bomb technicians. They thoroughly searched the interior of the plane as well as the cargo hold and all of the baggage," Eimiller said. A Thai Airways statement said the plane's return flight was delayed by several hours. The jetliner finally left Los Angeles at about 2:30 a.m. Wednesday, airport spokesman Albert Rodriguez said.