The men are accused of conspiring to destroy communications and other records about the May 29, 2020, killing of federal security officer David Patrick Underwood and attempted murder of his partner as they guarded the Ronald V. Dellums Federal Building in Oakland.
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The officers were shot while a large demonstration over the police killing of George Floyd in Minneapolis was underway nearby.
The four defendants agreed to plead guilty to obstruction of justice and will be sentenced early next year.
Another suspect, Steven Carrillo, 32, is accused of Underwood's killing as well as the fatal shooting of a Santa Cruz County Sheriff's Deputy.
Carrillo has pleaded not guilty and is awaiting trial.
The Associated Press contributed to this report.