The reward for information leading to an arrest for a Tokyo murder will double to ¥20 million, believed to be the biggest ever in Japan, investigative sources said on Wednesday.

The case involves the murder of a family of four in Setagaya Ward in late December 2000.

The original reward of ¥10 million includes ¥3 million offered by the National Police Agency and ¥7 million pledged by a private-sector organization.

The private group is planning to double the total, the sources said.

Mikio Miyazawa, 44, his wife, Yasuko, 41, and their two children were killed while sleeping in their home.

Previous large rewards include ¥10 million offered over a murder of three in 2002 in Matsudo, Chiba Prefecture, and ¥10 million for information on the slaying of a British woman in 2007 in Ichikawa, Chiba Prefecture.