Monahan said the note was a threat against him but would not elaborate.
This is the first action against the mayor in response to the city's direction that has gone beyond a complaint to the council from the dais, an internet message board or a paid advertisement.
The last time tensions appeared to boil over this much was St. Patrick's Day, when police were stationed outside of city leaders' homes through the night after a tumultuous afternoon. Earlier in the day, hundreds of Costa Mesa workers had been given layoff notices, an employee jumped to his death and Monahan chose to continue working at his bar instead of consoling employees at City Hall.
Lately though, the public backlash against the city has appeared to focus into a coordinated advertising campaign against Monahan and the council that could eventually lead to a recall.
Police said they have little to go on from Saturday's incident. The vandal apparently tried to throw the brick through two different bar windows first but missed before going to the front door. Detectives will have to review security video to see if they can get a look at the suspect.
It took officers at least 40 minutes to show up to Skosh Monahan's after the incident, officials said. Officers were dealing with a gang party on Darrel Street when police were called, said Sgt. Patrick Wessel.
Two employees were inside the business at the time and both were uninjured. Monahan estimated the vandal did about $1,200 of damage to his bar, but he said he'd have a more accurate number by Monday.