Austrian authorities were searching on Wednesday for answers to why a 21-year-old gunman shot dead 10 people in a rampage at his former high school before killing himself, one of the worst outbreaks of violence in the country's modern history.

Police said the man, armed with a shotgun and a pistol, acted alone. They are scouring his home and the internet for clues as to why he opened fire on the school in Graz, Austria's second largest city, on Tuesday, before shooting himself in a bathroom.

The incident was hard to properly take in, said a religious studies teacher at the school, Paul Nitsche, who left his classroom before the gunman tried to enter and briefly saw him trying to shoot the lock off another door.

"This is something I couldn't even imagine before," he told national broadcaster ORF. "As I ran down the stairwell, I thought to myself: 'This wasn't real.'"

Some Austrian media have said the young man, who has not been identified, apparently felt bullied, though police have yet to confirm this. Austrian authorities said the suspect never completed his studies at the school.

He left a farewell note that did not reveal the motive for the attack, police said, adding that a pipe bomb found at his home was not functional.

Ennio Resnik, a pupil at the school, said students and teachers needed time to come to terms with what had happened and asked that they be left in peace for a few days.

"It's surreal, you can't describe or really understand it," he said, speaking to reporters outside an events centre near the school where students were being offered counselling.

Some of the students gathered there cried, while others held each other.

Franz Ruf, director general of public security, said investigations into the motive were moving swiftly.

"We don't want to speculate at this point," he told ORF on Tuesday night.

Police were on the alert for potential copycat attacks, and they had received a threat against another school in Graz late on Tuesday, he said.

WATCH | Shooting suspect's motive remains unclear:

At least 10 killed in mass shooting at Austrian high school

Austrian police say a gunman killed at least 10 people in a mass shooting at a high school in Graz on Tuesday. The suspect, a 21-year-old former student of the school, killed himself. The motive remains unclear.

Around 17 minutes elapsed between the first emergency calls received by police about shots being fired at the school and the scene being declared safe, Ruf said.

Austria has one of the most heavily armed civilian populations in Europe, says the Small Arms Survey, an independent research project. The attack sparked calls for its gun laws to be tightened, including one from Graz's mayor.

Police said the guns used were in the suspect's possession legally, and Ruf said that while Austrian gun laws are strict, the case was being looked into. "If there are any loopholes, they need to be closed," he said.

Details of the attack have emerged slowly.

Austrian police said victims were found both outside and inside the school, on various floors. About a dozen people were injured in the attack, some seriously.

Austria declared three days of national mourning, with the shootings prompting a rare show of solidarity among often bitterly divided political parties. Parents of pupils and neighbours of the school struggled to make sense of the event.

Hundreds came together in Graz's main square on Tuesday evening to remember the victims. Others left flowers and lit candles outside the school. Dozens also queued to donate blood for the survivors.