A busy holiday weekend kept volunteer firefighters across Nassau County running from call to call, as nine fires broke out in just three days.

It's becoming more common for multiple fire departments to respond together to battle a single blaze. According to James Hickman, assistant chief fire marshal for Nassau County, every fire department in the county is staffed by volunteers and there simply aren’t enough of them to keep up with the growing demand.

“We've had 45 fire departments with well over 500 firefighters coming to the aid of their neighbors,” Hickman said.

He added that mutual aid has been critical during the recent stretch of fires.

“This is why you are seeing a fire department, wherever it is, getting two, three, four or five fire departments coming in mutually to their arms. They need the manpower,” Hickman said.

Investigators say four of the nine fires were caused by electrical issues. Another was sparked by burning debris, and one more was blamed on careless smoking.

The fires occurred in Valley Stream, East Rockaway, Seaford, Lynbrook, Wantagh, Franklin Square, Baldwin, South Bellmore and Mineola. 20 people were unable to return to their homes.

The American Red Cross on Long Island has stepped in to help families in need. CEO Jose Dominguez said the organization typically provides about $1,200 to a family of four.

“We average on a daily basis about one to two incidents per county. To see that in Nassau County in the matter of three days is pretty tremendous for our team,” Dominguez said.

Fire officials are urging residents to check their smoke detectors and make sure they are working properly, saying the devices can make all the difference in an emergency.