The United States Capitol Police arrested the suspect who allegedly “posted private, identifying information (doxing) about one of more United States Senators to the internet,” a statement said.
According to Legistorm, he worked for U.S. Sen. Maggie Hassan (D-NH) and previously worked for Sen. Barbara Boxer (D-CA). A September 2018 letter from Sheila Jackson Lee (D -TX) identifies him as a staffer for her office.
https://heavy.com/news/2018/10/jackson-cosko/