Verizon Wireless and a Kansas-based construction company said Thursday that they were responsible, through an as-yet-unnamed subcontractor, for work being done at the site of a massive natural gas explosion Tuesday that rocked Sun Prairie’s downtown, killing a volunteer firefighter and critically injuring another. Sun Prairie firefighter Ryan Welch was released from a hospital Thursday and brought to Fire Station 1, a block from the blast site, by a procession of emergency vehicles that were greeted by dozens of residents who cheered Welch into the station. At the station, firefighters and staff encircled Welch. Each took turns embracing their comrade.