Mayor Michael Bloomberg is calling for stricter rules for online gun sales after an investigation by his office found that background checks aren't done in a majority of Internet firearm transactions.

Bloomberg, the co-chairman of the Mayors Against Illegal Guns coalition, said 62 percent of gun sellers that use websites to sell their goods disregarded federal laws and sold the weapons with no questions asked. The group's two-year investigation also found one in 30 online buyers wasn't legally allowed to own a weapon.

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"If one in 30 people on a Boeing 747 were on a terrorist watch list, the plane would have 22 suspected terrorists on it -- something no passenger or airline would tolerate, and something that Washington should not tolerate, either," said Bloomberg, who also cited Monday's mass shooting at the Washington Navy Yard, which he said showed the need for background checks.

Bloomberg called on the federal government to develop laws that would enhance the Bureau of Alcohol, Tobacco, Firearms and Explosive's online investigation division and mandate background checks on all online gun sales.

"All of these steps would go a long way toward ending the kind of disturbing practices our investigation uncovered. And they might well prevent more needless bloodshed and loss of life in our city and nation," he said.