Tetsuya Yamagami, an unemployed 41-year-old, was identified by police as the suspect who approached Abe and opened fire during a campaign speech on Friday, an attack that was captured on video and shocked a nation where gun violence is rare.
Yamagami believed Abe had promoted a religious group to which his mother made a "huge donation", Kyodo news agency has said, citing investigative sources.
Yamagami told police his mother went bankrupt from the donation, the Yomiuri newspaper and other media have reported.
Tomihiro Tanaka, president of the Japan branch of Family Federation for World Peace and Unification, known as the Unification Church, told reporters at a briefing in Tokyo that Yamagami's mother was a member of the church. He did not give her name.
Tanaka declined to comment on her donations, citing the ongoing police investigation.
Neither Abe nor the man arrested for his shooting were members of the church, Tanaka said. Abe was also not an adviser to the church, Tanaka said, adding the church would cooperate with police on the investigation if asked to do so.
Abe appeared at an event hosted by an organisation affiliated with the church last September where he delivered a speech praising the affiliate's work towards peace on the Korean peninsula, according to the church's website.
Reuters was not immediately able to contact Yamagami's mother and could not determine whether she belonged to any other religious organisations.
Police have confirmed that the suspect said he held a grudge against a specific organisation, but have not named it.
Yamagami's mother first joined the church around 1998 but stopped attending in a period between 2009 and 2017, Tanaka said. About two to three years ago she re-established communication with church members and in the last half year or so has been attending church events at a frequency of about once a month, he said.
The Unification Church was founded in South Korea in 1954 by Sun Myung Moon, a self-declared messiah and strident anti-communist. It has gained global media attention for its mass weddings where it marries thousands of couples at a time.
Moon, who spoke fluent Japanese, launched an anti-communist political campaign in Japan from late 1960s and built relations with Japanese politicians, according to the church's publications.
Moon died in 2012. The church has about 600,000 members in Japan, out of 10 million globally, Tanaka said.Â
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