The construction site of a new Somerville movie theatre caught fire this afternoon, an incident that took place hours after firefighters doused two suspicious car fires, officials said.

At about 1:30 p.m., construction workers were doing welding work on the new building in Assembly Square when sparks dropped onto a pile of cardboard below, Somerville Fire Chief Kevin Kelleher said.

The fire department had a firefighter on scene in case of an incident of this kind. The firefighter was able to extinguish the blaze within 20 minutes, but did call for backup as a precautionary measure, Kelleher said.

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Had the firefighter not been on scene, the blaze could have become a two- or three-alarm fire, Kelleher said.

Earlier today, at about 1:15 a.m., two cars on opposite sides of Bow Street erupted in flames, Kelleher said.

The cause of the car fires is unknown. They are considered suspicious, Kelleher said, but unrelated to the suspicious fires over the summer. The cars, owned by Bow Street residents, were totaled, Kelleher said.