STOCKHOLM (AP) — Swedish artist Lars Vilks, who had lived under police protection since making a sketch of the Prophet Muhammad with a dog’s body in 2007, died in a weekend crash along with two police bodyguards, police said Monday. He was 75.

Vilks and two plainclothes police officers were killed in a head-on crash with a truck on Sunday afternoon, said Carina Persson, police chief for southern Sweden. All three died on the spot. The 45-year-old truck driver was flown to a hospital with serious injuries.

Persson said the police car, which had left Stockholm and was heading south, veered into the path of the truck and both vehicles burst into flames. The accident occurred near Markaryd, some 100 kilometers (60 miles) northeast of Malmo, Sweden’s third largest city.

“There is nothing else for now that indicates that it was something else but a traffic accident,” Persson told a press conference.

Sweden’s top police chief, Anders Thornberg, said an investigation would take place, but was expected “to take a relatively long time.”

Sweden’s Culture Minister Amanda Lind called called it “an extremely tragic traffic accident.”

Vilks was largely unknown outside Sweden before 2007, when he drew a sketch of Muhammad with a dog’s body. Dogs are considered unclean by conservative Muslims, and Islamic law generally opposes any depiction of the prophet, even favorable, for fear it could lead to idolatry.

Al-Qaida put a bounty on Vilks’ head. In 2010, two men tried to burn down his house in southern Sweden.

Since that time Vilks was forced to live under police protection. As Lind put it Monday, “due to the fact that he made use of his freedom of expression and his artistic freedom."

Over the years he continued to face threats to his life.

In 2014, a woman from Pennsylvania pleaded guilty in a plot to kill him.

The following year, a free-speech seminar that Vilks attended in Copenhagen, Denmark, was attacked by a lone gunman who killed a Danish film director and wounded three police officers.

Vilks, who was widely believed to have been the intended target of that 2015 attack, was whisked away unharmed by bodyguards. The gunman later killed a Jewish security guard outside a synagogue and wounded two more officers before he was killed in a firefight with police.

Police said Monday they did not know why the car drove into the wrong lane but they were investigating whether a tire might have exploded. The car transporting Vilks had puncture-proof tires, police said.

The accident happened on the north-south E4 highway Sunday afternoon. The unmarked police car apparently ploughed through a cable guardrail separating the north and south-bound lanes, senior police officer Stefan Siteus said at the news conference.

“We have found residues of tire on the E4 before the accident, and we are looking into the possibility that there could have been some kind of tire explosion,” Siteus said.

The vehicle weighed 4.5 tons, which could explain how the vehicle could drive through a cable guardrail, Siteus said. He added that the speed limit at the site was was 110 kilometers per hour (68 miles per hour). An ordinary large vehicle weighs less than 2 tons.

Born in 1946 in Helsingborg, in southern Sweden, Vilks worked as an artist for almost four decades, and rose to fame for challenging the boundaries of art through several controversial works.

His most famous pieces included “Nimis” — a sculpture of driftwood built without permission in Sweden’s Kullaberg nature reserve — as well as Prophet Muhammad drawings, including the one that showed the prophet as a dog.

Vilks initially planned to display the drawing at an exhibit at a Swedish cultural heritage center, but the drawing was removed over security concerns.

It went largely unnoticed until a Swedish newspaper printed the drawing with an editorial defending freedom of expression.

Several of his works, including driftwood sculptures and Mohammad drawings, including the one showing the Muslim prophet's head on the body of a dog, are currently on display in Warsaw, Poland. The works are being shown as part an exhibition curated by a right-wing director which aims to challenge left-wing political correctness.

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Jan M. Olsen in Copenhagen, Denmark, contributed to this report.