The National Police Agency asked NTT DoCoMo Inc. in March to develop technology to help investigators wiretap cellphone conversations, agency sources said.

The NPA urged the top cellular telephone firm to foot the bill for equipment and other costs, saying such a contribution -- estimated to reach up to 10 billion yen -- would be in the public interest, the sources said Sunday.

The agency said it wrote to DoCoMo simply to seek cooperation for investigative wiretapping based on a new law that takes effect today.

Under that law, enacted in August 1999, law enforcement authorities can ask communication companies for cooperation in such wiretapping.

A Justice Ministry official told the Diet in 1999 that such cooperation would be completely voluntary.

The sources said DoCoMo, which has a 60 percent share in Japan's cellphone market, has not responded to the request. Other cellphone firms have pledged to cooperate, the sources said.

The new legislation allows law enforcement authorities to use wiretaps to monitor private communications while investigating crimes involving drugs, guns and murders thought to have been committed by organized crime groups, and the mass smuggling of people into Japan.

Under the law, land line telephones in addition to cellphones, faxes and e-mail can be wiretapped.

Critics argue the legislation infringes on constitutional provisions regarding secrecy of communications and protection of privacy.