The attack on a girl’s elementary school in the Iranian town of Minab was one of the US military’s deadliest civilian bombings in decades. But nearly four months on, the Pentagon has produced no answers about why the military fired a Tomahawk cruise missile into a school on the first day of the war, killing at least 175 people, mostly children.

Some critics doubt that the Pentagon ever will, or will bury the results under classifications to keep the worst mistakes secret from the public.

As the US signs a shaky memorandum of understanding on a ceasefire with Iran, the secretive investigation into the attack has also become a test case for the self-styled secretary of war, Pete Hegseth’s new approach to what he calls “warfighting”. As he said in early March, nearly two weeks after the attack, “our rules of engagement are bold, precise and designed to unleash American power, not shackle it”.

Shortly after the attack, Donald Trump suggested that it was carried out by Iran. When it became clear that the strike used a US-made Tomahawk missile, he suggested that Iran also had access to the cruise missiles. It does not.

As he celebrated a ceasefire deal to open the Strait of Hormuz last week, Trump signalled he was ready to write off the attack as a mistake. “It’s such a strange question to be asked at this date, because you’re talking about a long time ago,” Trump said when he was asked about the investigation during a press conference at the G7 in Évian-les-Bains, France. “But nobody did that on purpose.”

It was at the beginning of what Trump has taken to calling a “little excursion” into Iran – that the back-to-back or “double tap” strikes on the school building took place, killing mainly children under the age of 12. Officials have told media anonymously that the site was believed to be an Islamic Revolutionary Guard Corps (IRGC) base.

Mohammadreza Ahmadi Tifakani lost two children in the school bombing. His seven-year-old daughter, Hanieh, was killed, along with all of her classmates in the girl’s section of the school when the first missile hit. According to witnesses, her 10-year-old brother, Sobhan, had survived the initial explosion, but ran back to look for his sister. He was killed in the second blast.

“I personally went to the morgue and identified both of them,” Tifakani told the Guardian in an interview shortly after the attack. “Sobhan was missing an eye, and half of his face was gone. His legs were broken. Hanieh’s skull was fractured, but her face was intact. I recognised Sobhan at first glance, even though he was severely injured.”

“Mistakes are made,” Trump said last week. “The war is nasty.”

Several former Pentagon and national security officials expressed doubt to the Guardian that the US government would take responsibility for the deaths of the schoolchildren in Minab – or even release the full report into the attack.

“It’s very rare that you would have a military operation and not have some incidents where there was a mistaken target and civilians are harmed or killed, but then there is a system for investigating, assessing accountability, and taking responsibility” in those cases, said one former senior Pentagon official.

“Even without the civilian harm mitigation office, there’s a very clear process for this, and I’m very doubtful that the Hegseth Pentagon will follow through,” the former official added.

As part of Hegseth’s “anti-woke” crusade at the Pentagon, the military has shuttered or reduced units meant to review civilian casualty incidents and more broadly indicated that decisions made in combat by “warfighters” would not be subject to such close scrutiny. The reduction in civilian oversight at the Pentagon under Hegseth may make it easier to skirt blame for the incident.

The incident is comparable to some of the worst mass-casualty incidents of past US wars, including the 2017 Mosul airstrike that killed at least 105 and perhaps more than 200 civilians, the 2015 Kunduz hospital airstrike that killed 42 people, and the 1991 Amiriyah air-raid shelter bombing that killed more than 400 Iraqi civilians who were sheltering during Desert Storm.

Trump said last week that the investigation was continuing. US Central Command, when asked about the investigation, gave no new information. “We have no updates at this time,” a defence official wrote.

But media reports indicate that the investigation has concluded. Preliminary results said the attack came because of the US using seven-year-old targeting data that failed to indicate that the building next to an IRGC base was in fact a girls’ school. The New York Times reported last week that at least one analyst had alerted a colleague several years ago that the US appeared to be targeting what was now a school in Minab. But the targeting data was not updated, and military officials continued to revalidate the site as a legitimate target for bombing.

Tifakani said at the time he had little hope of accountability from US investigations or the world. Asked what message he had for legal institutions or investigators into the bombing, he said: “They are witnessing everything themselves. We saw what happened in Gaza and Palestine. Now the same tragedy has befallen our own children. No matter what we say to them, that will not change anything.”

Congressional inquiries into the incident have also been stymied.

“The US strike in Minab is one of the most horrific episodes of the entire illegal Trump war in Iran,” Yassamin Ansari, an Iranian-American congresswoman who represents Arizona’s third district, said. She added that she had written to the Trump administration to demand answers about the strike and “gotten little to no response”.

“Donald Trump is hiding the truth from the American people and Congress, and deflecting blame to Secretary Hegseth, because he does not want the public to know the true horrors of what he unleashed on the Iranian people with absolutely nothing to show for it,” she added. “I will continue to do everything in my power to get answers for the families of these girls.”

Wes Bryant, a former US air force special operations targeting expert and former chief of civilian harm assessments at the Pentagon, said his few remaining colleagues overseeing civilian harm reduction at the Pentagon had been prevented from seeing the preliminary results of the investigation.

“I believe Hegseth and Trump are both going to do everything they can to suppress this investigation,” he said. “So, even if there is one really sitting there, it’s not getting out any more, unless we have, you know, a brave whistleblower.” He added that “the amount of people with eyes on that report are going to be very small”.

Strikes in Iran that had killed thousands of civilians were a sign of the rising “aggregate harm” that the US was willing to accept as part of a culture of that pointed to “pure negligence and recklessness, but also to a degradation of culture at senior leadership levels in the military”.

Early in his tenure as secretary of defence, Hegseth moved to close down or severely reduce civilian oversight of the Pentagon’s civilian harm mitigation and response and a report released in May by the department’s inspector general concluded the US military no longer had the people, tools or infrastructure needed to comply with two federal statutes requiring it to maintain a functioning civilian casualty policy, and operate a Civilian Protection Center of Excellence.

In September, Hegseth said publicly that he had done away with “stupid rules of engagement” for the US military as part of an anti-woke revamping of the Pentagon. In March, weeks after the strike on the school, as the US campaign against Iran continued at a fever pitch, he boasted that “warfighters have maximum authorities granted personally by the president and yours truly”.

Observers have said that the remarks and shuttering of key offices have limited civilian oversight at the Pentagon, with one former official saying that the US “threw in the trash the whole mitigating civilian harm strategy”.

Niku Jafarnia, the acting deputy Washington director for Human Rights Watch, said: “Hegseth himself has publicly expressed a lot of his scepticism around the amount of measures that we had in the military previously to mitigate these types of reckless errors and massive civilian harm incidents.

“He has publicly expressed scepticism about the value of constraints on fighters, and he has taken actions that have systematically weakened some of these protection measures that are supposed to ensure compliance with with the law.”

Pointing to Hegseth’s earlier public remarks about “untying the hands of our warfighters” and ignoring “stupid rules of engagement”, she added: “I think we saw the effects of that on day one of the war.”