The vast majority of the 658 single-pane windows at the U.S. Capitol were fortified during a renovation from 2017-2019, adding an extra layer of protection against attack.

But on January 6th, rioters somehow located the few windows that weren’t reinforced. Once broken, those windows served as main access points for the violent mob.

The Los Angeles Times, which first reported on the story, notes that it’s unclear if the vulnerable windows were located “by sheer luck, real-time trial and error, or advance knowledge by rioters.”

“Video shows some of the first rioters to break through the police line running past 15 reinforced windows, making a beeline for a recessed area on the Senate side of the building, where two unreinforced windows and two doors with unreinforced glass were all that stood between them and hallways leading to lawmakers inside who had not begun to evacuate,” reports The Los Angeles Times. The outlet adds:

The four unreinforced windows and doors that were the first points of entry on Jan. 6 are all in a recessed alcove, shielded by exterior walls on three sides. They were not the first windows, nor the easiest to reach for rioters storming up the Capitol steps. Attackers ran more than 100 feet across a courtyard to reach the covered outdoor entryway, where two unreinforced windows and one of the doors are.

The upgraded windows were not installed with riot prevention in mind. Instead, they were designed to withstand explosions. But they did thwart the rioters. “Every reinforced window that was attacked by insurrectionists appears to have remained intact, even if damaged,” reports the Los Angeles Times.

The windows that were not reinforced, about a dozen on the ground floor, were deemed low-risk – they were either in low-traffic areas or in places where authorities thought a potential bombing was unlikely.

But on January 6th, Trump supporters exploited their vulnerability.

“I couldn’t believe that they were able to breach those points as easily as they did,” Scott Lilly, a former House Appropriations Committee Democratic staff director, told The Times “It certainly shows that all of the money and inconvenience that we’ve instituted on those buildings was not particularly well spent.”

A House select committee is probing the January 6th attack. A spokeswoman for the committee told The Los Angeles Times that investigators “need to understand everything on that day.”