The court entry is a first step in obtaining a final judgment that could be worth as much as US$134 million.
"We've got the entry of default, that's pretty awesome," Laura Tanner, one of the plaintiffs' American lawyers, said from Salt Lake City on Friday.
The registration comes after Khadr, who was served with the lawsuit in prison in Alberta in June, failed to respond even though lawyers considering acting for him had twice requested time extensions to do so, court documents show.
"We haven't seen anything, no appearance of anybody, which is actually pretty strange," Tanner said.
Khadr's Canadian lawyers have previously said they were unable to find an American attorney to represent him at no charge.
The plaintiffs can now ask the court clerk for a default judgment. Alternatively, they could also ask a judge in Utah to hear the case, which could result in a total damages award of as much as US$134 million.
"We haven't really decided which path we're going to take yet," Tanner said.
In their lawsuit, Tabitha Speer and Sgt. Layne Morris allege Khadr, then 15, was responsible for the death of Sgt. Christopher Speer and Morris's injuries in Afghanistan in July 2002.
Morris has said the lawsuit, which seeks almost US$45 million, aimed to hold Khadr to account for his actions.
The suit leans heavily on Khadr's guilty plea to five war crimes before a widely maligned U.S. military commission in Guantanamo Bay in October 2010 that saw him sentenced to a further eight years in prison.
His plea deal included a stipulation of facts in which Khadr, now 28, admitted among other things to murdering Speer in violation of the rule of war and four other war crimes — although he has since said he only pleaded guilty to get out of American clutches.
While the Toronto-born Khadr is essentially broke, he is trying to sue the federal government for $20 million for alleged violations of his rights by Canadian intelligence personnel while he was in U.S. detention.
An American judgment could be enforceable in Canada but would involve having a Canadian court endorse it as the result of a fair process.
One of Khadr's Canadian lawyers, John Phillips, said he believed that would not happen.
"I find the whole thing unjust," he said.