Authorities say someone on the fifth floor of City Hall received a large envelope in the mail at about 8:30 a.m. That person opened it and saw what looked like "explosive-type material" inside.
The employee put the envelope in a stairwell and called 911. Between 300 to 400 workers were evacuated.
A hazmat team and a bomb squad responded to the scene. The device was rendered safe. City Hall resumed business at 1:30 p.m.
The device will be further analyzed to figure out what the substance was in the envelope. Authorities are also investigating who sent the package.
"We have no real problems that I'm aware of on the City Council. Nobody has made any threats or anything to us or the city as far as I know," said Huntington Beach City Councilman Jim Katapodis.
The fifth floor has offices for the fire department, economic development and community services. There were no reports of injuries or illnesses.
As a precaution, nearby Huntington Beach High School was placed on lockdown.