TOKYO, June 22 (Xinhua) -- Japan's Infrastructure Minister Keiichi Ishii told reporters on Friday that schools with similar walls to the one in Osaka, western Japan, that crushed a nine-year-old girl to death following its quake-triggered collapse should be inspected to make sure they are up to the ministry's standards.

Ishii said that owners of similar walls should conduct safety checks to ensure their structures are up to standard and take measures to upgrade them if they are not.

He urged the owners of the walls to follow the ministry's guidelines, which dictate that a wall's height must not exceed 2.2 meters and the wall should contain no cracks.

The guidelines also maintain that buttresses with reinforced iron rods at fixed intervals should support the concrete blocks.

Suspicions about the safety of the wall that crushed nine-year-old Rina Miyake have been raised, with experts saying that after the 6.1-magnitude quake hit the prefecture causing the wall to collapse and crush the young girl as she was on her way to school, it became apparent the wall's construction was flawed.

Local investigators are now looking into whether the cause of Miyake's death could have been because of professional negligence.

In Osaka, Takatsuki City is conducting inspections of concrete block walls at all of its 59 elementary and junior high schools, including Juei Elementary School where Miyake was a student.

It has also came to light that school authorities were warned about the substandard concrete wall at Juei Elementary School three years ago, sources with knowledge of the matter said Friday.

Disaster prevention adviser Ryoichi Yoshida apparently informed the girl's school and the local education board in 2015 about the potential danger of the wall, which was made from concrete blocks piled up high around the swimming pool of the girl's school.

In February, 2016, the Board of Education, said it tested the wall's strength and durability by hitting it with a stick and visually confirming that it was "safe."

The board said in a press conference on the matter that the two officials testing and checking the wall had no architectural qualifications and despite the girl's death said it did not deem the wall to be in violation of the law and was considered safe as there were no cracks.

Yoshida, who gave disaster prevention lectures at the elementary school on Nov. 2, 2015 after inspecting dangerous points about and around the elementary school, told the school's vice principal at the time that the concrete wall was unsafe.

His concerns were followed up with a detailed email including data to support his concerns about the wall, which he sent to the school on Dec. 7, 2015.

Yoshida warned the school that such walls built prior to the revision of the building standards law in 1981 could be particularly unsafe and remedial measures should be taken.

The education board, however, said Yoshida's advice was not passed on within the organization.

Japan's Education Minister Yoshimasa Hayashi in a press briefing on the matter Friday intimated that lax safety measures were taken regarding the collapsed 3.5 meter wall which was built of blocks piled higher than legal standards, and that investigations into the reason for its collapse would be verified.

"If safety measures were not taken despite warnings, we need to thoroughly verify the response," Hayashi said.

Osaka city authorities said that there are 17 schools with walls similar to the one that collapsed and of those, 15 have substandard structures that do not meet the law.

These walls will be removed within three weeks, they said.