Army Major-General Eldon Bargewell, who prepared the report, said the commanders were reluctant to give credibility to allegations that troops murdered civilians, and that no one took responsibility to investigate.
"There was evidence of an attitude that portrayed non-combatants as not necessarily innocents, which may have fostered a willingness to accept reported circumstances that might otherwise appear dubious," Bargewell wrote.
"A duty to inquire further was so obvious in this case that a reasonable person with knowledge of these events would certainly have made further inquiries."
He said the marine commanders had "sufficient knowledge and a duty" to report and investigate an incident after learning of allegations that troops had targeted civilians, but failed to do so.
"All levels of command tended to view civilian casualties, even in significant numbers, as routine and as the natural and intended result of insurgent tactics," he said in his findings.
Bargewell, himself a two-star general, was unable to question anyone who outranked him but recommended that "higher echelon commanders" be interviewed.
Major-General Stephen Johnson, the marines commanding general in western Iraq when the killings took place, told Bargewell he "didn't feel it was something significant in the sense that I had doubts about the conduct of the marines".
"Examples of many civilians being killed at a time were precedent for that," Johnson said. "It happened all the time."
"Examples of many civilians being killed at a time were precedent for that. It happened all the time"
Major General Stephen Johnson, Marines commander
The slayings occurred on November 19, 2005, when a roadside bomb destroyed a Humvee in a marine convoy, killing the driver.
A marine squad responded by using grenades and machine guns to kill 24 Iraqis, including women and children inside their homes.
A press release the day after said 15 civilians were killed as the result of a roadside bomb blast.
Intelligence reports at the time showed that, in fact, at least some deaths were caused by gunfire.
The public affairs officer who wrote the press release said he had "broad-stroked" the way civilians died because he did not want to get into graphic details, according to Bargewell's report.
The marines had extensive urban combat training, including house clearing procedures, but Bargewell said there remained a "level of confusion" about when to use deadly force.
He said some of the soldiers did not positively identify who they were shooting at, something that may be against military rules of engagement.
Three enlisted men are charged with unpremeditated murder and four officers are charged with dereliction of duty for failing to investigate the deaths.
The enlisted men say they are innocent and followed proper procedures, while the commanders claim they correctly reported and investigated the attack.
Preliminary court hearings are scheduled to begin next month at Camp Pendleton.
Dozens of marines in Iraq and the US were interviewed for the 104-page report.
A marine spokesman declined to comment on the findings, saying it would be inappropriate to comment on an ongoing investigation.