Hundreds more people died in Texas during the February deep freeze and blackouts than the state’s official count shows, according to an investigation by BuzzFeed News. The outages left millions of people without power as indoor temperatures dropped to deadly lows.

Texas has so far acknowledged 151 winter storm-related deaths. BuzzFeed News, on the other hand, found that an estimated 700 people lost their lives from the combined catastrophes of the storm and power outages. The news outlet conducted a thorough analysis of data that showed how many more people died around that time than would normally be expected. It’s the most recent analysis to show that the way authorities typically count death tolls from disasters tends to drastically low-ball the actual losses.

According to BuzzFeed News, the Texas Department of State Health Services counted mostly deaths from hypothermia, accidents involving ice, and carbon monoxide poisoning (people turned on cars and grills inside their homes in desperate attempts to stay warm). But hypothermia is “notoriously difficult to diagnose,” Hannah Jarvis, assistant medical examiner at the Harris County Institute of Forensic Sciences, tells BuzzFeed News. And the deaths of people who were already vulnerable to extreme cold and stress because of underlying medical conditions flew under the radar of Texas officials, BuzzFeed News reports.

Many of their death records point to those conditions, like cardiovascular disease or diabetes, as the cause of death. But family members and roommates who were there for their loved ones’ last, shivering moments say the freezing cold was to blame. “I still believe the cold made him to where his heart just gave out,” one grieving widow, Mary Gonzales, tells BuzzFeed News.

BuzzFeed News modeled how many deaths are expected in a given week, using long-term and seasonal trends. Then it compared that to actual deaths recorded by the CDC to find out how many more people likely died because of the storm and blackouts (and subtracted deaths from COVID-19).

Similar studies revealed how many people really died during Hurricane Maria and the blackouts that followed. Puerto Rico changed its official death count from 64 to 2,975 following public outrage over its initial underestimate and after a more in-depth report by George Washington University arrived at the higher number.

“We now know that the Texas government’s repeated failure to modernize our energy systems killed over 700 Texans in February’s winter storm,” former Texas representative Beto O’Rourke tweeted, calling for more action from lawmakers to harden the state’s energy system from climate change and more extreme weather. Many energy companies chose not to steel the state’s energy infrastructure against bouts of cold weather, even after a similarly brutal 2011 cold snap.

In the end, this story is about more than numbers. BuzzFeed News talked to families who are still fighting for justice after losing those they loved. Many were unable to qualify for support from FEMA because their death certificates didn’t mention the winter storm. Failing to officially acknowledge their deaths also puts lives at risk in future disasters, because public health officials aren’t able to see all the vulnerabilities that played out during the storm. BuzzFeed News shows what the state missed; read the full story here.