Japan began excavations Monday at a former army medical school to search for human remains linked to a notorious World War II program that allegedly conducted biological warfare in China and live experiments on foreign prisoners of war.

There is no certainty the excavation will unearth anything, but it is a sign that the government is open to the possibility of facing its long-kept wartime secrets, including the experiments conducted by the military's shadowy Unit 731.

Its activities have never been officially acknowledged by the government even though historians and participants have documented them.

Monday's excavation is the first government probe of the Tokyo site. It follows a former nurse's revelation that she helped bury body parts there as American forces began occupying the capital at the end of the war.

"If the bones or organs with traces of live medical experiments are found, the government would have to admit a wartime medical crime," said Yasushi Torii, head of a civil group investigating the case for decades.

"This is a start, although we probably need more evidence to prove Unit 731's role," said Torii, as he watched a shovelling machine and workers using hand-held shovels carefully dig part of the plot.

From its wartime base in Japanese-controlled Harbin in northern China, Unit 731 and related units injected war prisoners with typhus, cholera and other diseases that they mass-produced to research germ warfare, according to historians and former unit members. Unit 731 also is believed to have performed vivisections and to have frozen prisoners to death in endurance tests.

Historians estimate the number of the unit's victims from the thousands to as many as 250,000 _ mostly Chinese but they also may have included other nationalities. They believe some bodily remains of victims were transferred from China to Tokyo for analysis.

A Japanese court in 2002 acknowledged the unit's germ warfare in China but ruled that Japan had no obligation to atone to the victims. After years of denial, Tokyo acknowledged Unit 731's existence but has never disclosed details of its activities.

Unit leaders were spared prosecution in exchange for turning over information to the U.S., and given top posts in Japan's pharmaceutical industry, medical schools or the health ministry.

While the excavation was promised about five years ago under the previous conservative administration, authorities had held off until the scheduled relocation of residents and the demolition of apartments on the site last year.

The Democrats, which overthrew the conservative Liberal Democratic Party in 2009, have generally been "more open to the idea of taking an account of what happened in history and trying to turn a new page," said Koichi Nakano, a political science professor at Sophia University in Tokyo.

Health Ministry official Kazuhiko Kawauchi said the 100 million yen ($1.2 million) excavation is aimed at finding out if anything is buried in the plot.

"We are not certain if the survey will find anything," Kawauchi said. "If anything is dug up, it may not be related to Unit 731."

The former nurse, Toyo Ishii, now 88, broke 60 years of silence in 2006, saying she and colleagues at an army hospital at the site were ordered to bury numerous corpses, bones and body parts during the weeks following Japan's Aug. 15, 1945, surrender before American troops arrived in the capital.

Her disclosure led to a face-to-face meeting with the health minister at the time and a government pledge to investigate.

The site is near another area where a mass grave of dozens of possible war-experiment victims was uncovered in 1989 during the construction of a Health Ministry research institute.

Any remains found at the planned excavation site would have a stronger connection to Unit 731, experts say.

"The site used to be the research headquarters of Unit 731," said Keiichi Tsuneishi, a Kanagawa University history professor and expert on biological warfare. "If bones are found there, they are most likely related to Unit 731."

The 1989 find revealed dozens of fragmented thigh bones and skulls, some with holes drilled in them or sections cut out. Police denied there was any evidence of criminal activity.

The ministry concluded that the bones could not be directly linked to Unit 731. It said the remains were mostly of non-Japanese Asians and were likely from bodies used in "medical education" or brought back from the war zone for analysis at the medical school.