Danish Prime Minister Mette Frederiksen said today she “cannot rule out in any way that it is Russia” behind the Monday night drone incursion that forced Copenhagen Airport to shut down for four hours.

Frederiksen also told the Danish media that “what we saw last night is the most serious attack on Danish critical infrastructure to date.”

When asked about a possible motive, she said: “The obvious. To disrupt and create unrest. To cause concern. To see how far you can go and test the limits.”

At a press conference, Flemming Drejer, director of operations at Denmark’s PET intelligence service, said that national authorities are looking in all directions for a cause.

“But it is clear that, given the current threat and what we are seeing on the international scene, this is something we are aware of,” Drejer said in response to a question about Russia.

Russia sent drones over Poland earlier this month, several of which were shot down, as well as over Romania. Three Russian fighter jets were intercepted in Estonian airspace on Friday.

Anne Tønnes, director of the Copenhagen police, said it is something “extremely serious when you close an international airport for four hours.”

Tønnes announced that an extensive investigation has been launched in collaboration with national and international authorities, including those in neighboring Sweden and Norway, which also experienced a similar drone incursion.

Both Copenhagen and Oslo airports shut down late Monday night after drones were spotted in their airspace, forcing flights to be diverted or canceled and leaving tens of thousands of passengers stranded.