At first, students thought it was a Halloween prank.
The man walked into the Kronan school in Trollhättan, Sweden, on Thursday morning dressed like Darth Vader, with a long black trench-coat, black boots, black helmet and a mask similar to the "Star Wars" villain. He played eerie music but didn't say a word.
Strangest of all, he carried a sword.
Several students walked up to the man and snapped smiling photos on their cellphones.
"When we first saw him we thought it was a joke," a 14-year-old student named Laith told Swedish state broadcaster SVT. "There were pupils who wanted to take pictures with him and feel the sword."
What seemed like a Halloween joke, however, quickly turned into genuine horror.
When a teacher told the masked man to leave because he was scaring the students, the Darth Vader impersonator suddenly stabbed the 20-year-old teacher.
Then the attacker turned his weapon on the students, stabbing two teenagers before attacking another teacher, according to police.
"He hunted us through the school," another student told SVT.
The bizarre attack ended when police arrived at the school and fatally shot the intruder.
The young teacher died at the scene while a 17-year-old student later died at a hospital, police said, according to the Guardian. Two others, a 15-year-old student and a 41-year-old teacher, were also injured.
Neither the victims nor their attacker have been identified, although police did declare the incident a hate crime.
The attacker's choice of school may not have been random. Kronan has a high proportion of immigrants, and the killer allegedly singled out some of his victims, the AP reported.
Based upon his Nazi-like helmet, his manner of selecting his victims and material discovered in his Trollhattan home, the 21-year-old swordsman was motivated by racial hatred, police said.
"All together, this gives a picture that the perpetrator had a racist motive when he committed the crimes at Kronan school," police said in a statement obtained by the AP.
The apparently racially motivated killing spree was unprecedented in Sweden, a country that has long prided itself on civility and social cohesion but has recently seen an influx of immigrants as well as a surge in far-right, anti-immigrant sentiment. The attack is believed to be the deadliest ever to target a Swedish school.
"This is a black day for Sweden," Prime Minister Stefan Löfven said on Thursday, according to the AP. "It is a tragedy that hits the entire country."
Several Swedish media named the alleged attacker on Friday but his identity has not been confirmed yet by authorities. Social media accounts allegedly linked to the suspect show he liked German metal bands and videos about Nazi weaponry.
The attack caused panic in Trollhättan, an industrial city in western Sweden near Gothenburg. Terrified students, teachers and parents gathered outside the school on Thursday morning, according to the Guardian. Many returned after nightfall to light candles for the four victims.
The anti-racist organisation Expo said it knew the identity of the attacker and claimed he "showed clear sympathies with the extreme right and anti-immigration movements," according to the Guardian.
Police said the attacker entered the school through a cafe lobby that is open to the public before knocking on classroom doors. Eventually, the 20-year-old teacher opened a door to tell the masked man to leave.
"You are scaring the children, get away from here," the teacher said, according to Sara, the student.
The masked man "nodded, then he plunged the knife into the teacher's right side," Sara told SVT. "The last thing I heard him say was 'call an ambulance.'"
The swordsman then stalked students through the school, stabbing two of them and the second teacher.
"He was playing strange music and didn't say a word," Sara told SVT. "It was terrifying music, sort of Halloween music."
Prime minister Löfven urged his fellow Swedes to unite following the tragedy.
"What must never happen, has happened here today," he said at a press conference, the Local reported.
"We're going to embrace those we love [tonight] and think of those who no longer are able to do that," he said. "Take care of each other. Together we take care of Sweden."