A new letter from 74 lawmakers suggests that civilian casualties will become a focus of investigation if Democrats take control of Congress.

Democratic lawmakers are pressing the Pentagon to answer questions about U.S. airstrikes that killed civilians in Iran, in a possible preview of the party’s oversight agenda should it win control in the midterm elections next month.

The members focused on a strike in September in which three adults and two children at a wedding were killed. The strike also injured some 70 people.

In a letter to Defense Secretary Pete Hegseth, the House Democrats have demanded the Pentagon provide an account of the strike on Sept. 1 as well as other incidents in which large numbers of civilians were killed.

“This is the latest in an alarming pattern of civilian casualty incidents in strikes carried out by the United States in Iran since the war began,” the lawmakers wrote.

The House Democrats, for now, have little leverage to pressure the Department of Defense for answers. But should the party take control one or both chambers of Congress, the issue of civilian casualties in the Iran war could well become a focus.

The effort, led by Representatives Jason Crow of Colorado, Sara Jacobs of California and Yassamin Ansari of Arizona, had 74 signatures in all, demonstrating broad concern within the Democratic caucus.

The Pentagon has not acknowledged responsibility for the strike, saying only that an internal review is underway by U.S. Central Command. But a visual investigation by The New York Times found that the wedding, in Kuhestak, Iran, was struck by a bomb that had been released by American forces as they carried out intense attacks in southern Iran.

Neither the Pentagon nor Central Command, the department within the military with responsibility over the Middle East, has provided granular details of its military operations in Iran or of potential errant strikes that killed civilians. The department has also closed the area of the Pentagon where the press used to work from and placed tight controls on reporters’ entering the building.

In addition to the strike on the wedding, there have been a number of incidents of civilian casualties, most notably an attack on a Navy base in southern Iran in the opening hours of the war that destroyed an elementary school, killing at least 175 people.

The members of Congress said the number of strikes that had killed civilians was troubling, as was the lack of transparency from the Trump administration over the war more broadly.

“We are deeply alarmed by the continued pattern of civilian casualties resulting from U.S. operations in the war in Iran, and the apparent lack of action and accountability from the Department,” the lawmakers wrote in the letter. “Acknowledging such incidents, accepting responsibility, and learning from them makes our forces stronger, more effective, and an example to the world. When we fail to do so, it has the exact opposite effect.”

The letter from the lawmakers cited both the reporting from The Times and The Washington Post.

In the letter, the lawmakers ask the Pentagon if the U.S. military carried out the strike and what the intended target was. They also ask for details about any investigation into the strike in Kuhestak.

The lawmakers have also reinstated their earlier demand for information about the Pentagon’s review of the strike on the elementary school. More broadly, they are seeking information about another strike in Lamerd, Iran, on that same day, Feb. 28, that also reportedly caused civilian deaths as well as an accounting of other possible errant strikes.

The letter to Mr. Hegseth was sent on Oct. 8 and released publicly Sunday.

Asked for comment, a Defense Department spokeswoman reiterated an earlier statement from Central Command that the reports of the strike on the wedding originated on Iranian state media. “The U.S. military never targets civilians, unlike the I.R.G.C.,” the statement said, referring to Iran’s Islamic Revolutionary Guards Corps.

Mr. Crow, who helped lead the effort, is a former Army Ranger who served in the wars in Iraq and Afghanistan. Since joining Congress, he has consistently worked on the issue of harm to civilians in war. He was a driving force behind a law establishing the military’s Civilian Protection Center of Excellence and other measures aimed at minimizing errant attacks on civilians.

He has also collaborated with Ms. Jacobs on the issue of civilian casualties. In 2022, the two lawmakers were part of a group that founded a caucus in Congress to help protect civilians during conflict.

But Mr. Crow has been in the cross hairs of Mr. Hegseth’s Pentagon. He was one of six lawmakers who recorded a video telling U.S. service members they did not need to obey unlawful orders. President Trump accused the lawmakers of sedition and treason.