SAN JOSE — A Molotov cocktail was hurled at a teachers union building in East San Jose earlier this week, an official said, adding to a string of instances over the past couple of months where suspected vandals have inflicted damage on the site.

The fiery incident was reported Tuesday at 5:47 p.m. at the building in the 800 block of South Capitol Avenue, which houses the Mt. Hamilton chapter of the California Teachers Association and the Alum Rock Educators Association.

Jocelyn Merz, president of the Alum Rock union, said it appears that someone threw a Molotov cocktail — a World War II-era incendiary device typically consisting of a glass bottle of fuel and a soaked cloth for a fuse — onto the roof of the building.

San Jose Fire Department personnel responded and swiftly put out the ensuing blaze. Authorities said an arson investigator has been assigned to the case.

Merz said the apparent fire attack was just the latest incident where someone damaged the union building. She said on March 5, someone used a two-foot metal pipe to try and break into the front office window, followed two days later by someone leaving a propane tank — with the valve left open — under the same window.

The fire department also had to be called after the propane tank discovery to ensure the safety of the building.

On Jan. 29, Merz said someone threw cement rocks through floor-to-ceiling windows surrounding a conference room. At the moment, no one knows why the building has been targeted by one or more saboteurs.

“I have no idea. What I do know is that the building has been there for 40 years, with no incidents like this,” Merz said. “We are a little terrified to be in the building right now.

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Anyone with information suggesting suspicious elements for the Tuesday fire can contact the SJFD arson tip line at 408-27-ARSON, and tips about previous vandalism incidents can be made to Silicon Valley Crime Stoppers at 408-947-STOP, or at svcrimestoppers.org.