A number of Swedish government websites and major media outlets were knocked offline for hours over the weekend, police say. No one has taken responsibility for the cyberattacks, which silenced at least seven of Sweden's most prominent news organizations for hours amid growing tension with Russia.

A flood of web traffic Saturday night either crippled or totally shut down the news sites Dagens Nyheter, Svenska Dagbladet, Expressen, Aftonbladet, Dagens Industri, Sydsvenskan and Helsingborgs Dagblad for roughly three hours. Police launched an investigation Sunday, Agence France-Presse reported, with investigators telling many of the same sites the traffic appears to have originated in Russia.

Cyberattackers, ranging from Anonymous to state hacking groups, often use distributed denial of service, or DDoS, attacks to direct a wave of falsified web traffic at a single or small number of sites, overwhelming them with traffic for hours or days.

This attack was “extremely dangerous and serious,” Jeannette Gustafsdotter, the head of the Swedish Media Publishers' Association, told the news agency TT, as quoted by the Local.se. “To threaten access to news coverage is a threat to democracy.”

The onslaught came after an anonymous Twitter account, using the handle @_notJ, warned of imminent attacks against sites that posted “propaganda.” Aftonbladet, one of the sites mentioned in the tweets, has published a number of stories on the Russian plane crash that killed 62 people and other topics that don't portray Russia in a positive light.

This is what happends when you spread false propaganda.



Aftonbladet.se #offline@Aftonbladet — J (@_notJ) March 19, 2016

The following days attacks against the Swedish goverment and media spreading false propaganda will be targetted. — J (@_notJ) March 19, 2016

The attacks also came after a Swedish government report cited Russian “extreme movements, information operations and misinformation campaigns” aimed at Swedish lawmakers and the public as Sweden's most formidable intelligence threat. The Swedish government asked Russian Embassy staff to leave Sweden in 2015, though the report noted that suspected spies were still working as diplomats, airline employees and business executives.