COPENHAGEN, Denmark (AP) — A sword-wielding masked man who students thought was playing a Halloween prank stabbbed four people at a school with a large immigrant community in southern Sweden on Thursday, killing a teacher and a student before being shot dead by police.

The 21-year-old assailant, who posed with students before beginning his deadly rampage, roamed the halls of the Kronan school in the industrial town of Trollhatan, knocking on the doors of two classrooms and slashing those who opened them, authorities said.

The identity of the assailant was not released, though a police investigator, Thord Haraldsson, told reporters that a racist motive was “part of the picture.” He did not elaborate.

The attack stunned Sweden, where violent crime is relatively rare, though there has been a recent spate of arson attacks on asylum centers in the south of the country as the influx of migrants has surged. Immigration officials estimate that some 190,000 will arrive this year, second only to Germany in western Europe.

“This is a black day for Sweden,” Prime Minister Stefan Lofven said of the attack, the deadliest ever to target a school in Sweden. “It is a tragedy that hits the entire country.”

Laith Alazze, a 14 year-old student, said at first he thought the attacker — who was clad in black and wearing a helmet that masked his eyes — had something to do with Halloween.

“One of my friends walked over to him to challenge him, but when we saw he stabbed (the teacher), we ran away,” Alazze told Sweden’s TV4.

“He walked sort of like a soldier with a sword in his hand,” another student, identified only as Edona, told the station.

Panicked students fled the school, as police and ambulances rushed in. They found a dead male teacher and three people seriously wounded — two boys, aged 11 and 15, and another male teacher. One of the students later died of his wounds. None of the victims was identified.

Officers fired two shots, hitting the attacker with a bullet in the lower chest, and he died later at a hospital. 

Speaking at a news conference, Haraldsson said the assailant entered the school through a cafe in its lobby that is open to the public, then knocked on the doors of two classrooms, stabbing his victims. Police later searched the attacker’s home and found “interesting” things for their investigation, he added, without elaborating.

Several news outlets, including Dagens Nyheter, one of Sweden’s largest national newspapers, posted a photo of a black-clad man they identified as the attacker wearing a military-style helmet and a black mask with a sword in his hand. The media reports said the killer posed with two students before he started his rampage.

According to media reports, the school had held a meeting Thursday morning prior to the attack to discuss teachers’ fears they could not control access to the school as students had to go through the public cafe to reach the school’s cafeteria and other parts of the school.

Late Thursday, dozens of people gathered quietly to pay their respects outside the school, whose pupils range in age from pre-school to high school, many of them the children of immigrants. Some 20 percent of Trollhatan’s 56,000 inhabitants are foreign-born.

Lofven, who cancelled his scheduled program and rushed to Trollhattan, declined to comment on Swedish media reports that the attacker had right-wing sympathies, saying that police were still trying to establish his profile.

News of the attack brought expression of somber dismay. Sweden’s King Carl XVI Gustaf said the entire nation was “in shock” and that the royal family received the news “with great dismay and sadness.”

Sweden’s last school attack was in 1961, when a 17-year-old man opened fire at a school dance hall in the southwestern part of the country, wounding seven students, one of whom died later. Violent crime is rare in the country, which has strict gun-control laws. In 2013, there were 87 homicides reported in Sweden, a country of some 10 million people.