Over 2,000 suspected terrorists legally purchased guns in the last decade. All hail the 2nd Amendment.

Courtesy of the Washington Post:  

"Membership in a terrorist organization does not prohibit a person from possessing firearms or explosives under current federal law," the Government Accountability Office concluded in 2010. The law prohibits felons, fugitives, drug addicts and domestic abusers from purchasing a firearm in the United States. But people on the FBI's consolidated terrorist watchlist — typically placed there when there is "reasonable suspicion" that they are a known or suspected terrorist — can freely purchase handguns or assault-style rifles. 

And, as the GAO found, a number of them do: Between 2004 and 2014, suspected terrorists attempted to purchase guns from American dealers at least 2,233 times. And in 2,043 of those cases — 91 percent of the time — they succeeded. There are about 700,000 people on the watch-list — a point that civil libertarians have made to underscore that many on the list may be family members or acquaintances of people with potential terrorist connections. 

In other words to the question of whether or not what happened in Paris can also happen here, is a resounding YES.

However if, or when, it happens here the terrorists will be more heavily armed and their weapons will NOT have had to be smuggled into the country.