Late Thursday night in Dallas, snipers shot 12 police officers, killing five officers and injuring seven others, Dallas authorities said. The shootings came amid protests in the streets over police shootings that killed two black men in Louisiana and Minnesota this week.

These killings represent the largest single incident of police being killed since the 2001 terrorist attacks, and the largest single incident of law enforcement officers being shot to death since a sniper killed five officers in New Orleans over an eight-day period beginning Dec. 31, 1972, , according to records kept by the National Law Enforcement Officers Memorial Fund (NLEOMF).

Overall, officer deaths have become less common in America. In particular, shootings of officers peaked in the 1970s but have steadily tapered off since then. In the 1970s, an average of 127 officers per year were killed. By this decade, that number had dropped to 53, according to NLEOMF, which collects data on police officers who die on the job.

The decline is attributable to a number of factors, Peter Manning, a criminal justice professor at Northeastern, has previously told CNN. They include the improvement and wider use of bulletproof vests, more preparation for hostage and other high-risk situations, and the proliferation of specialized units to deal with major crises.

Thursday’s shootings, though, could represent a shift in the other direction. As of a Thursday update on NLEOMF's website, 25 officers have been killed by firearms, up 39 percent from the same date last year. This figure includes fatalities following the Dallas shooting, but not a fatality by friendly fire that occurred earlier this year.

Texas has historically ranked high for overall police fatalities — that is, by firearms or other means, such as traffic accidents. In 2015, it saw more such fatalities than any other state, 12 in total. And year to date, it leads in total fatalities with 10, including those following Thursday's shooting.

Of course, part of the reason is Texas’s size; it has the second largest police force in the country, behind California, according to FBA data. But population is not the only factor. California’s police force is 30 percent larger, but lost half as many officers as Texas’s in 2015.

Authorities reported that police in Dallas were targeted for killings, with the assailants perhaps "triangulating" their position. NLEOMF reports that "ambushed" officers represent 27 percent of shootings in 2015 and 41 percent in 2014. The next most common circumstance of an officer shooting, at a traffic stop, happened in 32 percent of shootings in 2015 and 19 percent in 2014.

Overall, 124 police officers died on the job last year, a number that is far below historical average but has recently ticked up. Most of those involve shootings or auto crashes.