BOULDER, Colo. (KDVR) — A fire that ripped through an apartment complex in Boulder last month has been deemed accidental by the fire department.

Fire investigators said the fire started from “discarded smoking material that was placed in a plant pot with potting mix and dried/dead plants.”

Twenty-four units were involved in the fire that was believed to have started on the upper floor and spread quickly through the attic at 2880 Kalmia Ave. on March 25.

Officials said when the incident happened, both the sprinkler system and smoke alarms activated. The Boulder Police Department and firefighters were able to evacuate everyone safely and no one was injured.

“This info can remind us that if you need to put something out that is or has been on fire, please ensure that it is completely out. Materials like potting mix may actually allow smoking materials to smolder and are not enough to extinguish flammable material,” Boulder Fire-Rescue tweeted.