ABC7 News has obtained video of them standing behind Vitalant Blood Donation, rummaging through the trash, taking something out the garbage bins, and then lighting it on fire.
The window melted, the wall is burnt, and the sprinklers went off inside, forcing the business to close today.
San Mateo Police Officer Michael Haobsh said, "This could have been worse. This could really have caught fire of that entire building. It could have spread."
Peninsula Family Service is right next door. It's also just a few yards away from another trash can fire the arsonists set in this parking garage.
Heather Cleary is CEO of Peninsula Family Service and said, "It's right in our backyard and this is where our staff are and we're concerned."
People who park in the garage on 2nd Avenue are scared.
Iris Delgadillo said, "There are cars, there could be an explosion, there's pedestrians, kids, there's a daycare next to us. Many things could happen. We're worried. What's going on?"
Investigators believe the arsonists left 2nd Ave. and walked down three blocks to 3rd and Ellsworth, where they set a trash can in front of Joseph A. Banks on fire.
While word of the arsons spreads throughout downtown, Heather Cleary is sure of one thing.
She said, "I'm concerned certainly. I won't sleep as well tonight as I did last night."