The Twin Peaks Republican was stopped at a security checkpoint early Wednesday morning, detained, and cited by airport police, said Nico Melendez, spokesman for the federal Transportation Security Administration.
Donnelly's carry-on bag housed a .45-caliber Colt Mark IV that had four rounds in its magazine and a spare magazine with five additional rounds, Melendez said. Lawmakers were returning to the Capitol today for the 2012 legislative year.
"When he was packing to leave, he had forgotten that it was in there," said Cassandra Joiner, Donnelly's spokeswoman. "He didn't realize it when he put it through the metal detector."
Donnelly -- a supporter of gun ownership -- was scheduled to board Southwest Airlines Flight 661, Melendez said. Passengers were to be transported from Ontario to Sacramento.
The incident sparked confusion because Donnelly's initial citation specified that the gun was unloaded. Ontario airport police later released a revised statement saying that the charge will be possession of a loaded firearm.
"It was an error on the officer's part," Sgt. Belinda Nettles, spokeswoman for airport police, said of the need to rewrite Donnelly's citation. She declined to discuss how the error occurred.
Possession of a loaded firearm is a misdemeanor, punishable by a maximum sentence of one year in jail and a $1,000 fine, Nettles said. Donnelly's weapon was confiscated by the Ontario Police Department. He will appear in court at a future date, she said.
Airline passengers legally can transport firearms via airline flights, but they must be unloaded and contained in a proper carrying case that is checked into the baggage department, not as a carry-on, Melendez said.
Donnelly would not be alone in forgetting that his carry-on bag contained a firearm -- 1,200 such incidents occurred nationwide last year, Melendez said.
Robin Swanson, spokeswoman for Assembly Speaker John A. Perez, urged against rushing to judgment in Donnelly's case.
"He deserves due process just like anyone else," Swanson said. "We support public safety. (Officials) will get to the bottom of it."
PHOTO CREDIT: Assemblyman Tim Donnelly, R-Twin Peaks, left, discusses his proposed measure that targets illegal immigrants during a rally at the Capitol in Sacramento, Calif., Monday, April 4, 2011. Associated Press/Rich Pedroncelli
* Updated at 12:04 p.m. to include information from Donnelly's spokeswoman, Cassandra Joiner, and from Robin Swanson, spokeswoman to the Assembly leader. Updated at 12:40 p.m. to add information about the ticket revision.