An estimated 16,000 computers in Japan may be infected with the same type of virus used to commit online bank fraud involving stolen user IDs and passwords, according to a police source.

The National Police Agency said it handled around 600 confirmed cases of unauthorized money transfers from 2011 through last month. Investigators suspect a single group may be responsible for many of the roughly 400 cases reported so far this year.

"It is possible that the number of cases will increase," an NPA official in charge of the matter said Thursday.

The virus, which has been spreading since early this year, is capable of stealing one-time passwords that are issued by financial institutions for use in a single transaction. The computers may have been infected when their owners accessed certain websites, according to investigators.

After analyzing the virus with the help of a computer security company, the police found that the stolen IDs and passwords were sent to multiple servers located in the U.S. and Europe.

The authorities identified computer terminals possibly infected by the virus from access records left in some of the servers, the source said.

The NPA has provided information on infected terminals to Internet service providers, asking them to notify users of the possible security breach and to remove the virus.