A former student opened fire at a school in Austria on Tuesday, killing 10 people and wounding at least a dozen others in the deadliest attack in the country’s modern history.

Students at the BORG Dreierschützengasse high school in Graz cowered in fear after the shooter — a 21-year-old alumnus with no previous police record — began the 17-minute rampage just before 10 a.m., police said.

The killer, who has yet to be publicly identified, used a pistol and a shotgun to shoot up two classrooms before turning one of the weapons on himself in a bathroom.

“Today is a dark day in the history of our country,” Austrian Chancellor Christian Stocker said.

Scenes from the chaos shared on social media show students cowering under their desks and yelling as gunshots could be heard coming from the hallways, with more than 300 officers — including special forces — racing to the campus.

At least nine people — six females and three males — were killed in the initial shooting, with officials not yet revealing their names or ages. A 10th victim died after being taken to a local hospital, Austrian news outlets report.

At least a dozen others were injured, according to local media, which said most were pupils.

The 21-year-old gunman acted alone and legally acquired his two guns, police said.  

He had been a student at the school, but hadn’t completed his studies, Austrian Interior Minister Gerhard Karner revealed.

Police allegedly found a letter, believed to be a suicide note, when searching the suspect’s house in Graz, Austria’s Kronen newspaper reported.

Unconfirmed reports said the shooter may have been bullied at the school.

Stoker, the chancellor, declared three days of national mourning, calling it “a national tragedy that shocks us deeply.”

“A school is more than just a place of learning,” Stocker said. “It is a space of trust, of security, of the future. The fact that this safe space was shattered by such an act of violence leaves us speechless.”

Austria has one of the most heavily armed populations in Europe, with an estimated 30 firearms per 100 people, according to the Small Arms Survey, an independent research project.

However, Tuesday’s shooting is the deadliest attack inside Austria since the end of World War II.

In 2020, four people were killed in Vienna by an Islamic State sympathizer, who also died.

A year before that, a 25-year-old man turned himself in for killing his ex-girlfriend, her family and her new boyfriend in the Alpine resort town of Kitzbuehel.

And almost exactly 10 years ago, a man killed three people and injured more than 30 when he drove an SUV through a crowd in downtown Graz.

With Post wires