Part of the Los Angeles Police Department’s headquarters in downtown Los Angeles was temporarily closed after a suspicious substance was found in a letter in the building’s mailroom. The substance was discovered shortly before 07:00 local time on Tuesday, when mailroom workers found a letter suspected of containing dangerous material, police officials said.

The workers alerted LAPD response teams and a hazardous materials unit was sent to the building. Mailroom staff were evacuated, along with adjoining areas including the police centre.

Fire units also closed roads around the headquarters at 100 W First St while officers investigated the letter.

Police said they were unable to identify the substance and did not provide further details about the suspicious mail.

Most of the headquarters remained open and a scheduled Police Commission meeting went ahead at about 09:30.

“The show must go on,” one officer said.

The meeting was attended by members of the public, although the crowd was smaller than usual. LAPD leadership provided updates on crime statistics and other cases during the meeting.

The headquarters is a 10-storey, 491,000-square-foot building that opened in 2009. It houses senior LAPD officials and executive and communications staff, as well as the department’s Compstat Command Center, holding cells and public assembly spaces.

The department’s growing fleet of police drones is also operated from the top of the building.

By 10:00, roads surrounding the headquarters had reopened and the building appeared to be operating normally. Activities at nearby Los Angeles City Hall also continued as usual.

A worker at the building said threats against the LAPD headquarters were not uncommon after the 11 September 2001 attacks.

Police are continuing to investigate the incident.