An envelope containing a suspicious white powder was found at the Social Security Administration office in Redwood City Monday afternoon, prompting a three-hour hazardous materials investigation, a Redwood City fire battalion chief said.

Firefighters responded to the building at 601 Allerton St. at about 3:35 p.m. and evacuated occupants as they worked to determine whether the chemical posed a threat, Battalion Chief Geoffrey Balton said.

Investigators concluded there were no dangerous chemicals present and cleared the scene by about 6:40 p.m., Balton said.

He said the investigation into the source of the suspicious envelope is being conducted by federal authorities.