Five bodies were found Wednesday in a car parked by a river in western Japan (search) in what apparently was the latest in a slew of group suicides, police said.

The four men and a woman, all of them believed to be in their 20s, were found slumped in their seats inside a car parked by a river in the town of Takashima (search), about 340 kilometers (210 miles) west of Tokyo, Shiga prefectural (state) police spokesman Takaaki Tanaka said.

Tanaka said the five people had no apparent injuries, and investigators believed they died in a group suicide. It was not immediately known whether they left a suicide note.

Police believe the five died of carbon monoxide poisoning from charcoal burned in a stove inside the car, with all windows tightly shut.

Japan has been the scene of several suicide pacts in recent years, many thought to have been plotted by people who met over the Internet.

So far this year, at least six groups totaling 25 dead have been found under similar circumstances, including the latest case.

Last year, 55 people in 19 groups committed suicide after meeting on the Web, up from 34 dead in 12 groups in 2003, the National Police Agency (search) said.

Suicide pacts have been made over the Internet since the late 1990s and have been reported everywhere from Guam to the Netherlands. Experts say they tend to occur in cycles, with news of group suicides prompting copycat incidents.