The National Police Agency said Thursday there were about 51,000 attempts by hackers to break into police computer systems throughout the country during the three-month period from July to September.

But agency officials said the attempts were not all necessarily directed at police, and that some of the hackers may have accessed the computers unknowingly because the agency does not disclose that police are in charge of the systems in question.

The NPA's Cyber Force unit began monitoring hacking attempts in April using a network linking 57 computer systems at regional police bureaus and prefectural police departments.