LOS ANGELES (CBSLA.com) — LAX Assistant Police Chief Brian Walker abruptly resigned Monday, according to an internal memo obtained by CBS2 News.

The resignation came just hours before CBS2 was to air an investigative report about allegations of corruption within the department that protects one of the nation’s busiest airports.

Walker admitted to misdemeanor tax fraud, and will be sentenced September 26. He faces up to a year in prison. The plea was part of a bigger case involving corruption and former Port of Los Angeles Police Chief Ron Boyd.

Since June, six former and active LAX police officers have come forward, accusing Walker of favoritism and rule-bending.

“I feel betrayed by my department,” said one former officer. “It’s too late for my case and then for other officers to be protected.”

“The running joke is, you have to get your third strike before you get your third stripe, meaning making sergeant,” said Marshall McClain, who represents the police officers’ union.

One allegations involves LAX Assistant Chief of Police Ethel McGuire, who was hired in 2010. Her resume includes a career at the FBI. But in 2013, state regulators notified the airport police department McGuire is not qualified to be a peace officer because she hasn’t been certified with the California Peace Officers Standards and Training (POST) commission.

A job listing for LAX Assistant Police Chief, posted in 2010, clearly states all candidates must be POST certified.

An airport police web page showed McGuire in uniform, until the CBS2 investigation began. The photo was then replaced with one showing McGuire in civilian clothes.

Walker wasn’t the only LAX Police official to resign just before CBS2’s report was to air. An internal affairs lieutenant who was part of the investigation also resigned Monday.

Officers who spoke to CBS2’s Cristy Fajardo are calling for a full investigation of the department.

“We need to find out how deep this rabbit hole goes,” said McClain.

The Los Angeles Airport Police Department released a statement, saying it “…takes all allegations of misconduct seriously, and has a process to investigate complaints.”