U.S. officials are investigating after a state senator boarded a flight to Hong Kong with an unloaded firearm in his carry-on luggage.

Washington state Sen. Jeff Wilson was arrested in Hong Kong on Friday after discovering the firearm in his carry-on luggage and reporting it to customs officials, according to a statement from Wilson.

A spokesperson for the Transportation Security Administration said Wilson boarded a flight from Portland and "passed through security and traveled with an unloaded firearm in his carry-on bag."

The TSA "takes this situation very seriously and is currently investigating the circumstances," the spokesperson said.

Wilson called the incident "an honest mistake" saying he discovered the weapon mid-flight between San Francisco and Hong Kong when he reached into his briefcase and felt the firearm inside. Once the plane landed, Wilson said he "immediately" reported it to customs officials.

In an interview with Oregon Public Broadcasting, Wilson blamed the TSA for not catching the firearm stateside.

"I should have never been out of security in America," Wilson said. The statement later added that "baggage screeners failed to note" the pistol in his briefcase while going through security in Portland.

The senator said he has since been charged in Hong Kong with possession of an unregistered firearm. He was released Sunday after posting bail and faces a hearing on Oct. 30, Wilson said.

Wilson said he was traveling with his wife on the first leg of a five-week vacation in Southeast Asia when the incident occurred.

TSA has previously reported its locating firearms in carry-on luggage at record rates in recent years. Earlier this month the agency said it's on track to intercept a record number of firearms at U.S. airports this year, catching an average of 20 guns a day.

ABC News' Karson Yiu contributed to this report.