Marking the 70th anniversary of one of Japan’s most compelling mass murder cases, a Tokyo gallery is showcasing 21 paintings made by a death row inmate who was convicted of the Teigin Incident, with the aim of providing the public with a look into the creativity of the award-winning artist who remained true to his craft even in prison while pursuing exoneration.

Sadamichi Hirasawa was accused of killing 12 people with poison and stealing money from a Tokyo branch of Teikoku Ginko (Imperial Bank), which was known as Teigin, on Jan. 26, 1948.

After first confessing to the murders, Hirasawa backtracked at the start of his trial, maintaining his innocence until he died in prison in 1987, at age 95, after nearly 40 years of incarceration.

No justice minister ordered him hanged during his incarceration. Efforts to overturn the death sentence have continued, with his lawyers filing a 20th appeal for a retrial with the Tokyo High Court in 2015.

The paintings displayed at Gallery Ten in Taito Ward are among thousands of works Hirasawa created in prison, from the 1950s to the 1980s, with painting supplies provided by his family so he could continue his career.

One painting shows a red flower against a background of iron bars, cats, Mount Fuji and the face of his daughter. Letters including his works, addressed to his supporters, are also on display.

Before his arrest, Hirasawa was known for his landscape paintings depicting the spaciousness of his homeland of Hokkaido.

“But in prison, surrounded by concrete walls and iron bars, he had to depend on his imagination and memory to create his art,” said Eizo Yamagiwa, who leads a civil group working to raise public awareness of the Teigin case. “Mr. Hirasawa must have felt great sorrow over his discomfort.”

The Teigin Incident stunned the nation, which was reeling in the wake of World War II.

“We have to continue making efforts to prevent memories of the case from eroding, because injustice will continue unless his innocence is proved in a retrial,” said Yamagiwa, a longtime human rights activist.

The civil group also aims to restore Hirasawa’s reputation as a painter. His lawyers, for their part, are appealing to the high court to reopen the case.

They have submitted a written opinion by a psychologist who casts doubt over the credibility of Hirasawa’s confession during questioning and the observations of researchers who closely examined witness testimony, including that of survivors.

The psychologist, Sumio Hamada, a professor emeritus at Nara Women’s University, argues that interrogation records indicate Hirasawa apparently struggled to cast himself as the culprit of the crime because he apparently knew few of the details.

The researchers concluded that the memories of the witnesses were affected by the flood of reporting about the arrest of Hirasawa and other factors when they were required to identify him several months after the crime.

The 12 victims were killed by a man identifying himself as a public health official, who claimed dysentery had broken out in the neighborhood and urged them to take a “remedy” that turned out to be poison. The man then escaped with cash and checks.

Hirasawa was arrested seven months after the incident, allegedly based largely on flimsy evidence.

The lawyers also plan to conduct biological tests in cooperation with doctors to pin down what kind of poison was used to kill the victims, based on the persistent doubts over the court determination that it was potassium cyanide.

“We will depend on leading-edge science so we can identify the poison,” said Ryohei Watanabe, one of the lawyers, expressing hope that the findings could overturn the final judgment and lead to the reopening of the case.

The admission-free exhibition of Hirasawa’s in-prison drawings is being held through Feb. 4, open from 11 a.m. to 7 p.m.