U.S. Border Patrol agents work in a field near Carrizzo Springs, Texas. (Eddie Seal/Bloomberg)

FOLLOWING MONTHS of damning disclosures about the use of deadly force by Border Patrol agents, Department of Homeland Security officials tightened the rules of engagement this spring. But it remains unclear whether U.S. Customs and Border Protection — with 43,000 agents, the biggest federal law enforcement agency — will end what appears to be a culture of impunity that has shielded agents from consequences and even meaningful investigations following senseless and unjustified killings.

A panel of outside experts reviewed 67 cases of the use of force by agents along the Southwest border, resulting in 19 deaths between January 2010 and October 2012. Its scathing report made clear that the agency often failed to carry out thorough investigations of the incidents.

Customs and Border Protection refused to release the report for more than a year; it was finally obtained this year by the Los Angeles Times. What the report called the “lack of diligence” in investigating incidents of force was all the more disturbing given its conclusion that some agents deliberately stepped into the path of fleeing vehicles to justify opening fire at them. In other cases, the panel found, agents had fired on people throwing rocks from the Mexican side of the border rather than de-escalating cross-border confrontations by seeking cover or retreating.

Mexican officials have long complained that Border Patrol agents are rarely punished or disciplined, even for shootings that appear wholly unjustified, and that the findings of internal investigations are buried for months or years. Investigations by the Arizona Republic, NPR and Washington Monthly, among others, have lent credence to those concerns while providing troubling details about seemingly unprovoked shootings by Border Patrol agents.

In one instance examined by NPR, agents fired across a border fence at Nogales, Ariz., repeatedly hitting a 16-year-old Mexican, Jose Antonio Elena Rodriguez; most of the shots hit him in the back. U.S. officials said the incident, in 2012, was prompted by rocks being thrown from the Mexican side; others have raised doubts about whether the agent who opened fire was in any danger. Nearly two years later, Border Patrol officials still have not released the results of any investigation or the name of the agent who opened fire. It remains unknown whether the agent has been cleared, disciplined or fired.

In an interview with NPR this month, the new Customs and Border Protection chief, R. Gil Kerlikowske, said he had initiated a review of scores of incidents involving his agents’ use of deadly force. Mr. Kerlikowske, a former police chief in Seattle and Buffalo, said bluntly that “we will be following through,” and he suggested he is determined to reform the agency’s culture. “There is a certain sense in law enforcement that if we just keep our heads down, all of this will go away — meaning media scrutiny and nongovernmental organizations,” he said. “That doesn’t happen.”

Those promising remarks have laid down a marker for an agency that has more than doubled in size in the past decade. Now Mr. Kerlikowske must be as good as his word.