The origin of handguns smuggled into Canada from the U.S. is often impossible to track due to a U.S. loophole that lets dealers sell guns without any paperwork at garage sales, auctions and gun shows.

Once someone sells a gun through that type of a private transaction in the U.S., police can no longer follow its trail, said Ottawa police Det. Chris Benson.

"'Cause there's no document and he hasn't broken the law," Benson said, "we can't trace that firearm beyond him. And after he sells it, that could exchange hands 10 times before it's used in a crime and recovered and maybe make its way up to Canada."

Licensed dealers in the U.S. have to file paperwork about a buyer and have a legal obligation to report suspicious transactions, aiding police in investigations such as a double shooting at Pari's Motel in Ottawa on June 21, 2007, that killed 28-year-old Phillip Salmon and 32-year-old Ziad Ahmad.

Jury selection was continuing Tuesday for the trial of Kwaku Frimpong of Ottawa who was charged with first-degree murder in those deaths. Frimpong was 23 at the time of the shooting.

N.B. resident convicted of gun-running

U.S. databases and the paper trail created when the .40-calibre pistol used in that homicide was legitimately purchased at a store in Maine allowed police to trace the gun to 32-year-old New Brunswick resident Andrew Porter, who is currently serving time in the U.S. after pleading guilty to gun smuggling.

However, no forms or background checks are required in many states for private sales.

After such sales, even a serial number is not enough to reveal where the gun came from, Ottawa police say.

Investigators said they were able to reconstruct the path of the Pari's Motel gun and trace about 30 others imported through the same network because Porter obtained the guns legally. An accomplice in Maine would go to a legitimate gun store and buy 10 cheap handguns at a time, leaving a paper trail.

Lawrence Sears, a 63-year-old man who is serving jail time for purchasing guns for Porter, told CBC News that Porter used the crossing at Milltown, N.B., which is smaller than the main Calais-St Stephen border post and doesn't have an X-ray machine, as the main crossing does. He also knew the customs officials well.

"He put 'em [the guns] under the seat and away he went to Canada with them," Sears said. "He knew what customs was on when he was going across the border."

Benson estimates that a cheap HiPoint handgun purchased new in the U.S. for $200 can fetch $1,000 to $2,000 in Canada, as their prices are marked up significantly due to their scarcity and illegality.