Washington DC - Democratic Senator John Fetterman once again bucked his party in refusing to sign a letter calling for an investigation into a deadly strike on a girls' elementary school in Iran.

"We write with grave concern regarding the airstrikes on a girls’ elementary school that reportedly killed at least 168 people – mostly children – in Minab, Iran during the opening salvo of U.S. and Israeli operations on Iran on February 28," Senate Democrats, along with independents Bernie Sanders and Angus King, wrote in a Wednesday letter to Defense Secretary Pete Hegseth.

"There must be a swift investigation into the strikes on this school and any other potential U.S. military actions causing civilian harm, and the findings must be released to the public as soon as possible, along with any measures to pursue accountability."

On February 28, dozens of people, many of them children, were killed when missiles fell on the Shajareh Tayyebeh Primary School in the town of Minab in southern Iran. The massacre came alongside wider US and Israeli strikes across the country.

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Analyses by CNN and The New York Times found the US was most likely responsible for the attack, while Human Rights Watch has called for a war crime investigation.

The lawmakers' letter asks Hegseth to respond to a series of questions, including on the intended target of the strike, any steps taken to prevent civilian harm, and whether any AI tools have been or will be used in Iran military operations.

Fetterman was the only member of the Senate Democratic Caucus not to sign on.

Fetterman continues to support the Trump administration's so-called "Operation Epic Fury," which Iranian officials say has killed more than 1,300 people to date.

"The United States never intentionally targets civilians, including its own citizens, unlike Iran. Everyone agrees it was a tragedy. Everyone agrees ​on performing a full investigation," Fetterman told Reuters of the school massacre.

A spokesperson added, "Whether the senator is on a letter or not, he fully stands behind a comprehensive investigation into this tragedy."

The Pennsylvania lawmaker, known as a staunch supporter of Israel, last week voted against a measure aiming to curb the Trump administration's authority to continue its attacks on Iran without congressional approval.