The justices ruled directly from the bench after a hearing on Thursday.
Anthony Barnaby and David Caplin are wanted by American authorities in a cold case investigation into the 1988 killings of Charlene Ranstrom and Brenda Warner. The two men were living in Nashua, N.H., at the time.
Barnaby was tried three times in the killings, but each trial ended in a hung jury and charges were dismissed after the third mistrial in 1990. Caplin was charged, but never tried after key evidence was excluded.
The case languished for two decades, but was re-opened in 2010, when police re-interviewed witnesses and new DNA testing techniques were used. The Canadian government ordered the extraditions in 2011, but they have been under appeal since then.
The Quebec Court of Appeal approved Caplin's extradition, but blocked Barnaby's, saying a fourth trial constituted an abuse and that the new evidence referred to by the American police was not really new.
Caplin appealed his extradition to the Supreme Court, while the Crown appealed the ruling that quashed Barnaby's extradition. The two cases were heard together.
Mark Sisti, the American lawyer who defended Barnaby 25 years ago and worked on his extradition case, said he's stunned that the case is being resuscitated.
"It's absolutely outrageous," he said in an interview. "I can't even imagine this thing coming back here.
"I don't know what's going on here. I am absolutely befuddled as to why a case as weak as this one has drawn so much attention from the attorney general's office in New Hampshire."
Sisti said no one in New Hampshire has ever been tried four times for the same crime.
The Crown argued before the Supreme Court that a fourth trial might be unusual, but was not necessarily an abuse.
"Fourth trials have been allowed in recent years in Canada," the Crown said in its factum.
The Crown also said it wasn't up to the Court of Appeal to decide whether the latest evidence was new or not, since it did not have all the facts that would be available to an American trial judge.
"It should not have been open to the Court of Appeal to decide if the evidence was new evidence in the context of a foreign trial."
Sisti said none of the supposedly new witnesses are actually new.
"They were all known, we knew all these people," he said.
Since the nine justices ruled right after the hearing, their reasons for sending the two men back were not immediately available.
Sisti said the two men may be extradited, but there's a long way to go before they ever get in front of another jury.
"What's being set up now is extensive pre-trial litigation, reconstruction of the case itself, reopening all of these avenues of investigation, reinterviewing everybody, transcribing and delivering all of the transcripts of all three trials which will be approximately three to four months of testimony which has to be transcribed ... it's going to go on for quite a while."
Putting things back together will be hard, he said.
"You have witnesses that have died. All of our police reports and notes and transcripts burned in a fire at my office in 1994."