Japanese authorities say a cyberattack against government websites last year was a distributed denial of service, or DDoS, attack.

The National Police Agency has been investigating why access to some websites, including the "e-Gov" portal, were temporarily blocked. Websites of some private firms were also disrupted.

The results of an analysis jointly released by the agency and the Cabinet Secretariat's national cybersecurity center show up to 100 gigabytes of data per second was sent from tens of thousands of IP addresses. They also show most of the addresses were abroad.

Police suspect a group of hackers illegally operated a large number of communication devices to launch the attack. They are looking into possible links between the incident and pro-Russian hacker group, Killnet.

The group has claimed responsibility for the attack.

It is rare for Japanese authorities to release the results of such analyses while investigations are still underway.

The authorities hope to prevent similar attacks from causing damage, by urging government offices and private firms to step up cybersecurity. They recommend using a system that identifies suspicious access from abroad and setting up a distributed network of servers known as a content delivery network.