STOCKHOLM – When darkness fell on Tuesday night, Stockholmers looked at the sky and wondered if the drones were on their way back.

In recent days, reports have multiplied of black-forced machines flying overnight over a range of sensitive sites, including three nuclear power plants, a large water treatment plant and a royal palace.

A photograph published by Expressen on Tuesday – shot by a passer-by, Mathias Øgendal – showed what appeared to be a drone that flew over central Stockholm’s amusement park Gröna Lund.

“I saw it through the front windshield of my car, flying high over the city,” Øgendal told the newspaper. “I thought it was weird that it flew around there.”

As a sign that the reports are taken seriously by the authorities, the Security Police (known as Säpo) have taken over the investigation of the flights over the nuclear power plants.

Per Engström, police chief in Stockholm, told SVT on Wednesday morning that no suspect has been identified so far, but that the investigations continued and the police were still open to any motives behind the drone operations.

“There is a lot of speculation about motives – that it could be an attempt to map places, for example,” he said. “It could also be people who have flown their drones to these places by mistake.”

But as the number of reports of drones has increased, and so have eyewitness accounts suggested the machines were of a winged type favored by the military, speculation has grown that a hostile state may be responsible.

With relations between Russia and Western nations – including Sweden – as tense as they have been for decades, speculation has revolved around Moscow and whether drone flights can in any way be linked to the deteriorating regional security picture.

“It’s an obvious thought that arises, but it’s impossible to judge, you can only speculate about it, and people do,” sa Martin Hagström, head of research at the Swedish Defense Research Agency. “There’s a reason why it’s important to investigate this.”

The Russian embassy in Stockholm did not respond to an email requesting comment on speculation that there may be a connection between Russia and the drone observations.

The first drone observations were reported on Friday evening over the Forsmark nuclear power plants – just north of Stockholm – and Ringhals and Oskarshamn in southern Sweden. In the days since then, similar observations have been made over the Royal Palace in Drottningholm and a large water treatment plant in Norsborg – both on the outskirts of Stockholm – and the Government Offices in the center.

While the origins of the drones remain a mystery, it is clear that the aircraft arrived at an unusually tense time for Sweden.

Last week, the country increased its military presence on its strategically important Baltic Sea island of Gotland amid reports of unusual naval activity by Russia in nearby waters.

Accusations and denials

An era of growing Swedish suspicions about Russian military and intelligence activities probably began in 2013, when a group of military planes flags near the Swedish coast in what was later judged to be a simulated attack on Stockholm.

In 2014, a submarine was judged to have entered water near Stockholm and triggered a massive sea hunting operation. Russia denied involvement, but senior Swedish lawmakers pointed the finger at Moscow.

Sweden has had problems with mystery drones before. In 2016, a military training exercise was stopped when mysterious unmanned aircraft were saw over your head.

Swedish authorities said that the investigation of these latest drone flights is ongoing and comments from police chief Engström indicate that some progress is being made.

“We have begun to see some patterns in how they behave around certain facilities so we hope to get closer to the truth in different ways,” he said.