The suspect was last seen walking toward the Interstate 5 highway from the mall before police arrived, Francis said, adding: "We're actively searching for suspect, tracking leads, etc."
The mall was being evacuated.
"EMS starting to enter to attend to injured inside Mall w/ police escort and after initial clearance," Francis said on Twitter shortly before 8:30 pm (0330 GMT), referring to emergency medical services.
"Once EMS gets in, holding out hope that the number confirmed goes down."
Francis said survivors would be transported by bus to a nearby church.
Burlington is located about 70 miles (110 kilometers) north of Seattle in the US Pacific Northwest state.