According to police, witnesses noticed two men running from Strangnas Cathedral before jumping into a small, white speedboat and escaping on Lake Malar on Tuesday (local time).
The robbers walked into the small town’s medieval cathedral in broad daylight and made off with priceless crown jewels dating back to the early 1600s, police said.
Two gold-plated and jewel-encrusted crowns and an orb dating from the 17th century were reportedly taken.
Swedish police searched overnight for the suspects but “did not result in any significant breakthroughs in the form of suspects or detentions,” they said on Wednesday.
The theft has now been registered with international criminal investigation organisation Interpol, enabling an international search.
The two men vanished after the heist into a vast patchwork of lakes around Strangnas, 60 kilometres west of the capital Stockholm around noon on Tuesday, police said.
The crowns were part of the funeral regalia of Charles IX, son of Gustav Vasa, who was buried in the cathedral in 1611.
The stolen items were on display at an exhibition in the cathedral, and visitors were inside at the time. Police could not put a value on the stolen jewels.
“The alarm went off when the burglars smashed the security glass and stole the artefacts,” Catharina Frojd, a spokeswoman for the 14th-century Strangnas cathedral, told The Associated Press.
The church wrote on its website that the stolen items were kept “in accordance with the prevailing safety regulations in locked and alarmed displays in the cathedral”. It gave no further details.
Police sent out a helicopter and boat to hunt for the thieves but found nothing. Authorities said nobody was hurt in the robbery.
While the items are of great historic and cultural value, police expressed doubt whether the burglary would bring the perpetrators financial gain.
The stolen items are “impossible to sell” because of their uniqueness and high visibility, Maria Ellior of the Swedish police’s National Operations Department told Swedish news agency TT.
“Images are being shown in the media. It’s simply not possible to sell these kind of items. So you can only wonder what their intentions are, and how much they know about these crowns,” she said.