One glance at the run-down block in central Tokyo’s Bunkyo Ward may suggest nothing but derelict industry, easily passed by, but in reality it is where Japan engaged in its own abortive version of the Manhattan Project.

The nondescript buildings belie this little known history and the feverish race toward the end of the war by an embattled Japan to produce what was hoped would be a “decisive weapon” — an atomic bomb.

It was here at the former site of Rikagaku-kenkyujo, or Riken, the Institute of Physical and Chemical Research, that the nation’s top scientists under the direction of Yoshio Nishina faced increasing pressure from the government and military to make a nuclear bomb for use against the United States.

Earlier this year, letters and documents belonging to Nishina were published in three volumes. Covering several decades of his career, they feature documents and diaries describing the nuclear weapons project.

The letters and other documents that make up “Nishina Yoshio Ouhuku Syokan Syu,” published by Misuzu Syobou in Tokyo, were found recently in Nishina’s office, which has been preserved in the old block that was the former site of Riken.

After World War II, the U.S. military investigated his team’s effort and concluded “the level of research in nuclear energy development in Japan had not advanced beyond the university experimental level.”

Nishina agreed sometime in 1940 to a request from the Japanese Army Aviation Technical Institute to initiate the atomic weapon project.

He and his group, however, were aware of their disadvantages, including scarcity of uranium deposits and well-trained scientists. In a 1943 meeting with the navy, Nishina said it was so difficult to utilize nuclear energy that “even the United States cannot make it during this war.”

The documents reveal that the most difficult part was being able to enrich enough uranium to make a bomb. This is still the prime challenge for nations with nuclear ambitions.

In his diaries, Masashi Takeuchi, one of Nishina’s disciples, discussed the misplaced enrichment efforts. They tried only the thermal diffusion method, which was thought by the United States to be the most difficult option, and gave up on other methods.

In March 1944, the Nishina group erected a two-story thermal diffusion tower to separate uranium isotopes at the Riken site as the public became hopeful that a secret but decisive weapon could be developed to turn the war in Japan’s favor.

That year the Japanese science fiction novel “San Francisco is Blown” was published, portraying how a Japanese scientist succeeds in making an atomic bomb that is subsequently dropped on San Francisco.

A U.S. air raid in April 1945 destroyed the tower, dashing even the smallest chance for enriching uranium.

Ryohei Nakane, a junior member of Nishina’s group, said in an interview, “Dr. Nishina recruited young physicists not to send them to China or Southeast Asia as soldiers.” Nishina is thus credited with keeping many highly educated young Japanese out of the war.

The three-volume collection also raises intriguing historical questions. In a letter to Nishina dated April 21, 1933, a German physicist disclosed that Edward Teller was hoping to stay in Japan after fleeing Nazi Germany.

A Jewish scholar, Teller knew he could not continue his studies in Germany and hoped to continue his research in Japan. Japan was still seven years away from joining the Axis alliance.

If Teller, the leading figure in the U.S. development of the thermonuclear bomb, had joined Nishina’s group, would Japan have been the first to produce the bomb?

A few days after the atomic bombings of Hiroshima and Nagasaki, Nishina was sent to both cities, where he acknowledged that the two bombs were actually the kind he had sought to develop. Writing a colleague, Nishina discussed losing the atomic race.

“It may be time for members of the group to commit ritual suicide,” he wrote. “U.S. scientists won a great victory over we Japanese Riken scientists. In the end, they have had higher morale than us.”

He would later become a strong opponent of nuclear weapons.

In the two cities he saw “burned human bodies everywhere and countless people who are severely wounded and lying on the ground helplessly.” As a top physicist who led Japan’s efforts to build the bomb, Nishina felt a deep need for atonement.

He urged a “nonwar policy” and encouraged scientists to persuade politicians not to develop nuclear arms.