Hackers targeted the websites of telephone operators in the Czech Republic on Thursday after similar attacks this week affected major banks and leading media companies.
Police are investigating the unusual wave of attacks that started on Monday, and are cooperating with their colleagues from other countries in their probe, said police spokeswoman Marketa Johnova.
The two major telephone operators in the country—Telefonica Czech Republic and T-Mobile—said their websites were down Thursday due to the attacks.
Ctibor Jappel, spokesman for Telefonica said the company's website was hit by the denial of service attacks, a technique that works by overloading a website with online traffic. Jappel said that Telefonica IT experts have established that the attacks originated in Russia. He said the website resumed operations in less than an hour because the company was ready for them.
T-Mobile's website was down for more than two hours.
Internet safety expert Ivan Svoboda told the Czech public television that it is "very unusual" that nobody has claimed responsibility for the attacks so far.
Explore further: Dutch police investigate apparent hacker attack