A sustained distributed denial-of-service attack is being blamed for dropping Congress' central website from the web for the past three days.
The Library of Congress (loc.gov) and the US Copyright Office (copyright.gov), which are operated on the same servers, also struggled to stay online.
The Library of Congress said in a tweet that technical staff "are working hard to restore full service", and apologized for the inconvenience.
As of Wednesday morning, the sites appeared to be up and running.
The three websites are widely used for accessing data relating to congressional and copyright matters, such as bills and committee notes. The Sunlight Foundation, a nonpartisan and nonprofit organization that pushes for government transparency, said in a blog post that the downtime is "another reason why opening legislative data to the public matters," so that other sites, like Govtrack, can keep the public informed during downtime.
But so far, no group or hacker affiliation has taken credit for the attack.
Federal news site FCW reported on Monday that the attack was likely carried out using DNS reflection, which makes it tough to trace the attacker's identity.
This kind of attack can flood a website by amplifying the amount of traffic that's sent by the attacker. Reflection attacks can increase the volume of attack traffic by dozens of times over.
DNS remains the most prevalent protocol for conducting large-scale and powerful distributed denial-of-service attacks, according to Arbor Networks.
The strongest publicly recorded attack was said to be upwards of 480Gbps.