A woman working at an office building across the street from a mass shooting at a Nashville Christian school described seeing police cars speeding to the scene Monday morning.

Jozen Reodica told CNN she feared the worst after seeing cruisers pouring into the parking lot of the Covenant School, where a female shooter slaughtered three children and three adults before being shot down by cops.

“It all happened so fast,” she told the outlet, noting police quickly cordoned off streets in the area near the elementary school as word of the atrocity began to spread.

Reodica, who works at Shearwater Health directly across from the school, filmed frightened students being led out of their building by police.

She said they walked in a single-file line while holding hands.

Authorities said a 28-year-old woman had carried out the carnage, but have not yet identified a motive.

Shots first rang out at 10:13 a.m. and the assailant was dead at 10:27 a.m. after officers’ prompt response, officials said.

The killer is believed to have been a former student at the private Christian academy.

The shooter was armed with at least two assault-style rifles and a handgun.

Cops said there is video of the incident from inside the school, which is being reviewed.