Japan's most senior policeman began an embarrassing climbdown Thursday after his officers arrested four people over cyber threats issued when their computers were apparently hacked.

Emails containing threats to attack targets including a school and a kindergarten attended by Emperor Akihito's grandchildren were sent from infected computers in different parts of the country.

The computer owners were arrested and held, in one case for several weeks, in a system where custody conditions are harsher than those in other developed countries and where police rely heavily on confessions.

Two of the suspects reportedly admitted sending the emails before a broadcaster and a lawyer received an anonymous message containing information investigators conceded could only have been known by the real culprit.

The message said its sender had taken control of several personal computers to send other threats.

"There is a high possibility that we have arrested people who are not actually criminals," Yutaka Katagiri, the head of the National Police Agency, told a news conference, while adding their innocence was not certain.

"If it is found to be so, we will take appropriate action, including issuing apologies to those who were wrongly arrested," Katagiri said, adding that police would introduce "more cautious measures" for tracking down cyber crime culprits.

Japan's legal system prizes confessions, and prosecutors are generally unwilling to take on cases without the suspect having first acknowledged his guilt.

Suspects can be held for a total of 23 days before they must be charged or released.

It is not uncommon for police initially to arrest someone on a lesser charge and then re-arrest them on suspicion of a more serious crime just before the 23 days is up.

Critics charge that this system is open to abuse and gives police the right to hold people without charge for long periods. They also say the use of intimidatory tactics by officers is not uncommon.