WASHINGTON (AP) — Contractors who built the web portal for the Obama administration's health insurance marketplace said Thursday the site's crippling problems trace back to insufficient testing and changes that government officials made just prior to going live.

Who's to blame? The first congressional hearing into what went wrong dug into issues of website architecture and testing protocols — but also re-stoked the partisan battle over President Barack Obama's signature expansion of health coverage for millions of uninsured Americans. Republicans who've been trying to kill the program the past three years sounded outraged that it is being poorly carried out, while Democrats jeered them as political hypocrites.

What was clear after more than four hours of testimony was that the contractors had only partial answers, and only the Obama administration can eventually put the entire picture together to explain the botched rollout.