(CNN) -- It has been called the most popular rifle in America, and it briefly returned to the spotlight after Monday's shooting at the Navy Yard: the AR-15.
A U.S. law enforcement official said Monday that gunman Aaron Alexis unleashed a barrage of bullets using an AR-15, a rifle and a semi-automatic handgun. Authorities believed the AR-15 was used for most of the shooting, the official said. The news prompted Sen. Dianne Feinstein, one of the strongest proponents of a ban on assault weapons like the AR-15, to issue a statement the same day asking, "When will enough be enough?"
However, federal law enforcement sources told CNN Tuesday that authorities have recovered three weapons from the scene of the mass shooting, including one -- a shotgun -- that investigators believe Alexis brought in to the compound. The other two weapons, which sources say were handguns, may have been taken from guards at the Navy complex.
The sources, who have detailed knowledge of the investigation, cautioned that initial information that an AR-15 was used in the shootings may have been incorrect. It is believed that Alexis had rented an AR-15, but returned it before Monday morning's shootings. Authorities are still investigating precisely how many weapons Alexis had access to and when.
Regardless, the massacre pushed the AR-15 back into the gun-control debate. The weapon has been used in several other rampages that shocked the nation:
-- Sandy Hook: Adam Lanza killed 26 people at the Sandy Hook elementary school in Newtown, Connecticut, with an AR-15 in December 2012.
-- Aurora: Police say James Holmes killed 12 people and wounded 58 using an AR-15 rifle, a 12-gauge shotgun and at least one of two .40-caliber handguns police recovered at the scene.
-- Portland: Jacob Tyler Roberts stole an AR-15 and killed two people in a mall food court in December 2012.
-- Santa Monica: John Zawahri allegedly pieced together an AR-15-type gun and went on a rampage that started at his father's home and ended at Santa Monica College in June. Five people were killed.
More or fewer guns
Advocates for tighter gun laws don't want to see the semi-automatic cousin of the military-issue M4 assault rifle in the hands of civilians.
"Almost every mass shooting involves an AR-15 assault rifle," Staff Sgt. Alonzo Lunsford told CNN's Piers Morgan Monday night.
"It's the preferred mass shooter's weapon of choice," added Lunsford, who was wounded by Maj. Nidal Hasan during the Ft. Hood shooting in 2009.
"But I don't see a logical reason why any civilian needs to have one of these killing machines."
Gun ownership proponents, on the other hand, say the solution lies in more guns, not fewer.
That's precisely the point CNN commentator and pro-gun activist Ben Ferguson made to Morgan.
Armed security guards aren't enough, he said. Once a gunman gets past them, they are no longer effective.
"When you allow a military base to be infiltrated by an individual like this... at that point he has free rein," Ferguson said.
Had the contractor and civilians working in the Navy Yard been armed, they could have gunned down the gunman and stopped the shooting quickly.
Morgan, who has dedicated many hours of his show to gun violence in the United States, did not seem hopeful that there will be a resolution soon to the controversial issue, even after the latest slaughter.
"I want the day to come where we don't have to have this ridiculous debate time and again in America," he said in frustration. "I just cannot have this debate anymore. It is ridiculous."
Efficient but deadly
The AR-15 is an efficient killing machine that originated as a U.S. Army rifle, military documents say. At the time, it was a fully automatic weapon.
The Army tested it as early as 1958.
It was said to be more effective than its predecessor, the M14, and showed distinct advantages against the AK-47, the weapon carried by the United States' opponent in the Vietnam War, the Viet Cong.
It led to a surge in military purchases during the mid-1960s. Eventually the AR-15 was further developed into the M16 rifle.
The original M16 patent ran out years ago, and now the AR-15 is manufactured by several gun makers.
The AR-15 has since become a semiautomatic rifle, firing one bullet per squeeze of the trigger. But like the M16, ammunition is loaded through a magazine.
In the Sandy Hook Elementary School shooting, police say Lanza's rifle used numerous 30-round magazines.
An AR-15 is usually capable of firing a rate of 45 rounds per minute in semi-automatic mode.
Under the 1994 federal ban on such weapons, buying some variants of new AR-15s was against the law. The ban expired in 2004.
It's easy to get an AR-15. They can be purchased online or in a gun store. Online purchases are sent to a gun store for pick-up and a background check is still required.
Early Tuesday, one online retailer had a special running: Plunk down $999, and it can be delivered to the gun retailer of your choice in 10 days.
The gunfire in the nation's capital quickly drew echoes from Washington politicians.
Some, like Feinstein, renewed their calls for restrictions on semi-automatic weapons.
The California Democrat, one of the strongest proponents of a ban on assault rifles, called for Congressional action. "We must do more to stop this endless loss of life," she said.
The new mass shooting is nothing new, bemoaned Democratic Sen. Jay Rockefeller of West Virginia. "We are becoming far too familiar with senseless, tragic violence. This is the seventh shooting since 2009, and these repeated incidents demand our attention," he said.
But their words will likely fade away, said Democratic Sen. Joe Manchin of West Virginia. The broad support for gun ownership in Congress is too strong.
He doesn't think the shooting will be enough to sway the 60 votes a gun control measure would need to break a Republican filibuster.
Efforts to push gun control legislation through Congress led by Vice President Joe Biden after the Sandy Hook tragedy have run aground.
The National Rifle Association did not respond to a request for comment Monday. The gun rights organization has typically not responded to similar shootings immediately.
CNN's Dana Bash and Bryan Koenig contributed to this report.