The New York Times reviewed hundreds of documents including police reports, bank records and investigator notes from a decade of mass shootings. Many of the killers built their stockpiles of high-powered weapons with the convenience of credit. No one was watching.

Two days before Omar Mateen killed 49 people and wounded 53 more at the Pulse nightclub in Orlando, he went on Google and typed “Credit card unusual spending.”

Mr. Mateen had opened six new credit card accounts — including a Mastercard, an American Express card and three Visa cards — over the previous eight months. Twelve days before the shooting, he began a $26,532 buying spree: a Sig Sauer MCX .223-caliber rifle, a Glock 17 9-millimeter semiautomatic pistol, several large magazines, thousands of rounds of ammunition and a $7,500 ring for his wife that he bought on a jewelry store card. His average spending before that, on his only card, was $1,500 a month.

His web browsing history chronicled his anxiety: “Credit card reports all three bureaus,” “FBI,” and “Why banks stop your purchases.”

He needn’t have worried. None of the banks, credit-card network operators or payment processors alerted law enforcement officials about the purchases he thought were so suspicious.

Mass shootings routinely set off a national debate on guns, usually focused on regulating firearms and on troubled youths. Little attention is paid to the financial industry that has become an instrumental, if unwitting, enabler of carnage.

I am not sure that there is a way to use this information without further prying into the personal lives of everyone, but this is yet another data point about how the culture of guns in the United States is fundamentally broken.