The deadliest mass killing by a single shooter in U.S. history, carried out by a man who legally purchased weapons used in the attack just days before the crime, is adding to the heated debate on gun violence.

U.S. President Barack Obama has tried to encourage more background checks for gun sales. But even as he announced an executive action last January that would, among other things, require vendors at gun shows to be licensed in the hopes they'll then conduct background checks, he admitted he still needs Congress to act.

After gunfire killed 50 people, including the gunman, at an Orlando, Fla., nightclub popular with the city's LGBT community, Obama called the shooting "a further reminder of how easy it is for someone to get their hands on a weapon" that lets them target large groups of people.

Here are key figures on gun violence and ownership in the U.S.

Mass shootings

San Bernardino shooting

Police officers conduct a manhunt after a mass shooting in San Bernardino, Calif., on Dec. 2, 2015. Gunmen opened fire on a holiday party at a social services agency, killing 14 people and wounding 17 others. (Mike Blake/Reuters)

There were 330 mass shootings in the U.S. in 2015, according to the Gun Violence Archive, a non-profit organization founded by Washington businessman Michael Klein. The archive defines a mass shooting as four or more people being shot and/or killed in a single event, excluding the shooter.

No official government source tracks mass shootings in the U.S. and the number can vary based on different definitions. For example, the website Mass Shooting Tracker previously included shooters in death tolls, which resulted in a total of 355 after the mass shooting in San Bernardino, Calif. It said the final number for 2015 was 371.

The same database says there have been 176 mass shootings so far in 2016, following the Orlando massacre.

Gun ownership

Obama Guns Colorado

Fifty-seven per cent of young Americans who took part in the GenForward survey support a ban on rapid-firing semi-automatic weapons, with support especially high — 74 per cent — among Asian-Americans. (Cliff Grassmick/Daily Camera via Associated Press)

The number of guns owned by civilians in the U.S. added up to 270 million, according to a 2007 estimate from the Small Arms Survey, a Geneva-based organization that collects data on small arms and armed violence. That is approximately 89 guns per 100 U.S. citizens and more than one gun for each adult. This represents about 42 per cent of the world's total civilian gun ownership, despite the U.S. having less than five per cent of the world's population. No other country has more civilian-owned guns than the U.S. 

However, the number of U.S. households reporting firearm ownership is decreasing. A report from National Opinion Research Centre shows 31 per cent of households surveyed said they had a gun in 2014 — tied with 2010 for the lowest in the past 40 years. The peak for this figure was in 1977, when 50.4 per cent of surveyed households reported gun ownership.

Gun-related homicide rate

The rate of firearm homicides in the U.S. was 3.3 per 100,000 people in 2009, the most recent year for which comprehensive data was available, according to the United Nations Office on Drugs and Crime (UNODC).

This rate is greater than those of many countries featured in the UNODC data, including Canada (0.5) and the U.K. (0.1). Most European countries have rates within the range of the U.K. and Canada.

However, the U.S. didn't have the highest rate of firearm homicides in the UNODC data. The rate in Honduras was 57.6 per 100,000 people. Several other countries also had higher rates than the U.S., including Mexico (7.9) and Colombia (28.1). 

Data from the Pew Research Centre shows that the U.S. firearm homicide rate has decreased from its peak of 7.3 in 1993, but has remained fairly steady since 2000.

Homicides by firearms

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FBI agent Ron Hopper appears with police and community leaders in Orlando to talk to reporters about the shooting that left 50 dead and 53 injured in the early morning hours of June 12. (Gerardo Mora/Getty Images)

The number of firearm homicides in the U.S. for 2014 was 10,945, according to the Centers for Disease Control and Prevention.  

Firearm homicides in the U.S. have been steady since 2000, usually hovering at 11,000 to 12,000 per year.

Numbers compiled by the Gun Violence Archive show that so far in 2016, gun violence in the U.S. has killed 5,967 people and injured 12,251 others.

Suicide

The number of suicides by firearm in the U.S. in 2014 was 21,334, also according to the Centers for Disease Control and Prevention. 

Suicide represented 66 per cent of gun-related deaths in 2014. Firearm suicide rates have been on a steady increase since 2006, when there were 16,883. 

In total, the U.S. had 42,773 suicides in 2014, with a suicide rate of 13.41 per 100,000 people. Suicides by firearm make up nearly 50 per cent of that figure, with a rate of 6.69 per 100,000.

By comparison with latest available statistics, Canada had a suicide rate of 11.3 per 100,000 in 2012 — but only 1.4 of those were by firearms.