The Asahi Shimbun, a well-read liberal newspaper in Japan, reported last week that some Finance Ministry documents related to the dubious deal implicating Abe and private-school operator Moritomo Gakuen, appeared to have been revised before they were submitted to the Diet for scrutiny after the scandal emerged last year.
The report, citing a few anonymous sources, said that large portions had been deleted from the original documents, as well as phrases that describe the heavily-discounted land deal as "special" and "exceptional."
According to local reports, revising the documents could constitute a violation of law on management of official documents.
Upon repeated requests from opposition parties, the Finance Ministry had promised to submit on Thursday copies of the original documents to a meeting involving senior members of the budget committee of the House of Councilors.
But the documents submitted on Thursday turned out to be almost the same as the ones shown to the parliament last year and the ministry declined to clarify whether there were other documents existing beside those submitted.
The opposition lawmakers, accusing the ministry of trying to cover up the scandal by not showing the original documents, boycotted the subsequent parliament session, while the ruling bloc continued the upper house budget committee deliberations with most of the opposition lawmakers absent.
"This is not simply a matter of whether documents were altered or not. It is a matter that destroys the very foundation of politics," said Renho, an upper house lawmaker from the opposition Constitutional Democratic Party of Japan.
Abe, for his part, said the Finance Ministry should "make maximum efforts" to explain the situation soon.
Deputy Prime Minister and Finance Minister Taro Aso, a close ally of Abe, said that the ministry would make all the efforts it could to investigate the case.
Moritomo Gakuen, a private-school operator, reportedly bought a 8,770-square-meter piece of land in June 2016 in Toyonaka, Osaka prefecture, for 134 million yen (1.26 million U.S. dollars), equivalent to only 14 percent of its appraisal price.
The land had been intended for a new elementary school to be opened in April last year with the prime minister's wife Akie as its honorary principal, though she has since stepped down as the scandal widens.
The prime minister has since denied that he or his wife was involved in the shady deal, while Yasunori Kagoike, head of the school operator, gave sworn testimony in both chambers of parliament, stating that he believed the land deal involved "politicians' intervention."
The scandal has for one time drastically brought down the Abe administration's support ratings, and according to multiple media polls, the majority of those surveyed believed that the government had not given sufficient explanations on the scandal.