An 18-year-old male student was killed at least seven others were wounded on Tuesday after two students walked into a Denver-area school and opened fire, law enforcement officials said.

Several of the victims of the shooting were in critical condition and undergoing surgery at local hospitals, Douglas County Sheriff Tony Spurlock said at an afternoon briefing, according to Reuters.

Spurlock declined to comment on a possible motive for the shooting rampage, saying it was too early in the investigation to speculate.

Earlier, the Douglas County Sheriff’s office said deputies heard shots coming from the Science, Technology, Engineering and Math (STEM) School in Highlands Ranch, located about 25 miles (40 km) south of Denver.

The STEM school, which is kindergarten through 12th grade, was on lockdown on Tuesday afternoon and all other Douglas County schools were on lockout as well.