Just how easy is it to buy a gun in America?

The deadliest school shooting in US history has fueled a simmering debate on gun control -- an emotive issue with strong voices on both sides.

Herewith a fact box on federal, state and local laws governing gun ownership, which the pro-gun lobby insists is a right enshrined in the US Constitution under the second amendment.

-- Federal law requires licensed gun sellers to conduct a background check on anyone buying a gun. The law prevents convicted criminals, drug addicts, domestic abusers and the mentally incompetent, among others, from buying guns.

The so-called Brady Law was named after James Brady, one of late president Ronald Reagan's assistants, who was left disabled after being shot in an assassination attempt on Reagan in 1981. The law came into effect in 1994.

The law has been credited with preventing hundreds of thousands of weapons from passing into criminals' hands.

Loopholes in the law allow private dealers at gun shows in certain states to sell weapons without having to conduct background checks.

Such shows can operate on a "no questions asked, cash-and-carry" basis.

The current system makes it easy for criminals and even juveniles to buy as many guns as they want at gun shows, including assault weapons, according to the Brady Campaign to Prevent Gun Violence.

Sellers are not required to keep records on gun show sales, making it almost impossible for police to trace such weapons if they are used in a crime, according to the campaign group.

-- The Gun Control Act came into force in 1968 after the assassinations of civil rights leader Martin Luther King and New York senator Robert Kennedy. It bans the sale of firearms across state lines, except for licensed dealers.

-- A federal law banning the sale of semi-automatic assault weapons came into effect in 1994 but expired in 2004.

-- State laws vary widely, with some preventing anyone under the age of 18 from buying any kind of gun, making adults criminal responsible for weapons used by juveniles, or banning instant gun sales to prevent crimes of passion.

Other states require owners to undergo special training, demand firearms be registered or have laws banning the carrying of concealed weapons.

In the state of Virginia, where Monday's university shooting happened, a 12-year-old can legally buy an assault rifle, even without parental permission, every 30 days, according to the Brady Campaign. Other states have no minimum age limit for those wanting to buy rifles and shotguns in particular.

-- A third level of legislation exists at local and city level, for example in New York City, which has some of the most restrictive gun laws in the United States, only allowing certain permit holders to own guns.

-- The second amendment of the US Constitution, which asserts that "the right of the people to keep and bear arms shall not be infringed," is often at the center of any attempt to introduce new gun control legislation.

While the pro-gun lobby argues that any attempt to control gun ownership potentially undermines a constitutional right, some of those in favor of tighter restrictions say the amendment is an anachronism that was meant only to refer to militia at the time the document was drafted more than 200 years ago.

bur/jah/vs