Emergency personnel rushed to the scene, in the 400 block of Stanford Drive, at about 11:45 p.m. Thursday after receiving a call about a possible weapon offense, said Lt. Chris Cammock of the Mount Vernon police.
Officers discovered an explosion had ignited a fire and blown out a large window in an apartment unit. Flames and smoke were billowing from the apartment.
The building was evacuated as firefighters attacked the blaze.
No one was found injured at the scene, but a short time later two men and a woman in their 20s arrived at Skagit Valley Hospital suffering from burns. The injuries do not appear life-threatening.
The Washington State Patrol's bomb squad responded to make sure the apartment unit was clear of any explosive devices.
Police evacuated the entire building for about two hours until it was determined that the areas not affected by the explosion or fire could be reoccupied. The Mount Vernon Fire Marshal determined that five units surrounding where the incident occurred will remain closed until the investigation is complete.
Investigators believe the occupants were attempting to extract hash oil from a marijuana plant, Cammock said. The process involves mixing parts of the marijuana plant with butane gas.
In this case, a two-liter pop bottle was being filled with butane gas. Police believe the butane fumes filled the apartment when an ignition source - possibly a water heater - sparked the explosion.
Cammock said he has never seen this type of incident before in Mount Vernon.
"It may be just more common because ... the legalization of marijuana, that it's a little more prevalent in that use. It's another method for that type of use both consumer use and recreational use," he said.
The force of the blast shattered a large window and damaged exterior walls of the apartment unit. Damage estimates were not immediately available.
Charges could be filed against the three injured people.