PHOENIX -- The Gilbert Fire Department is still investigating a fire that charred five construction vehicles Sunday, authorities said.

The vehicles -- one water tanker, three flatbed trucks and one pickup truck -- belonged to CPC Construction Inc., a Gilbert construction company that has been in operation for 17 years.

"It's always upsetting because you wonder how it happened, why it happened," said Troy Colby, the owner of the company.

Investigators said they don't know yet how the fire started.

Colby said the business will reopen Tuesday.