Earlier this month, the board hired Judge LaDoris Cordell to investigate complaints against the sheriff's department just three days after holding a no-confidence vote in the department's chief of staff. She recently interviewed Monaghan.
The Board believes the Sheriff Christina Corpus became aware of the interview shortly before firing Monaghan.
"The timing of the Sheriff's action and information provided by Assistant Sheriff Monaghan suggest possible retaliation against Assistant Sheriff Monaghan for participating in Judge Cordell's independent investigation," Mueller said.
Corpus released a response, denouncing the board's statement and describing it as "completely out of line".
The San Mateo County Attorney's Office is reviewing the situation, according to the board's statement. They also requested Cordell investigate whether Monaghan suffered retaliation for participating in this independent investigation.