The Intercept was provided a copy of the letter before was publicly released. You can read it for yourself here. In it, they ask Bezos what law enforcement agencies are using the software, who Amazon has offered Rekognition to, information about error rates and bias and how Amazon is making sure its software isn't used to further systems that disproportionately impact protected groups. The letter requests answers by June 20th.
There's a lot of concern about the use of this kind of tech by law enforcement officers. Specifically, who and what governs how it is used? Could it be used to track people based on race, or to unlawfully follow immigrants? This is what privacy groups are concerned about, and likely why House Democrats are asking Bezos for answers.