The gunman who entered a gay nightclub in Orlando early Sunday — killing at least 49 people and injuring 53 others in the worst mass shooting in U.S. history — was “well-armed and well-prepared,” according to the police, who are calling the attack a “terror incident.” The gunman was identified by law enforcement officials as Omar Mateen, 29, who lived in Port St. Lucie, Florida, according to The New York Times.

In the coming days, there will be a lot of discussion about terrorism and mass shootings and the relationship between these categories. And it’s growing harder to separate them: Terror attacks in the U.S. increasingly involve guns.

Although terrorism still accounts for a negligible share of all gun deaths in the U.S. — since 1970, fewer than five deaths most years — from 2002 to 2014, 85 percent of people killed by terrorists in the U.S. were killed using guns, according to our analysis. Every terrorist attack in the U.S. last year in which someone other than the perpetrator was killed involved guns, according to a preliminary list provided by Erin Miller, who manages the Global Terrorism Database. Since the Sept. 11 attacks, the number of people killed by guns in terror attacks in the U.S. has risen, as has the number of terror attacks involving guns.

The attack in Orlando would be the deadliest terrorist attack in the U.S. since Sept. 11, eclipsing the mass shooting in San Bernardino, California, in December, which left 14 people dead, and a shooting rampage at the Fort Hood, Texas, military post in 2009, in which 13 people were killed. The second-deadliest terrorist attack in the U.S. last year was a shooting at a Charleston, South Carolina, church that killed nine people. More people died in the Orlando attack than in all U.S. terrorism attacks using firearms in any single year since at least 1970. (Terrorists killed 44 people in gun attacks in 1973.) The police have not identified a motive in the Orlando attack yet, but if the shooter targeted the nightclub, Pulse, because it was a gay club, the attack — like the Charleston shootings — would be classified as a hate crime.

The San Bernardino and Charleston attacks were two of the three deadliest mass shootings in the U.S. last year. The third, carried out at Umpqua Community College in Roseburg, Oregon, killed nine people.

Experts say the increased use of guns in terror attacks is an alarming trend. Arie Perliger, director of terrorism studies at the U.S. Military Academy, said that U.S. terrorists are turning to guns because since Sept. 11, the federal government has monitored the use of explosives and the trade of materials that can be turned into explosives. People on the terrorism watch list aren’t barred from buying guns, by contrast, although such a ban probably wouldn’t have stopped the Charleston or San Bernardino shootings, because the suspects weren’t on the watch list.

Guns are easier for terrorists to work with than explosives “and are less likely to result in a terrorist operation being compromised,” said Jeffrey Simon, a visiting lecturer in the department of political science at the University of California, Los Angeles, and author of the book “Lone Wolf Terrorism: Understanding the Growing Threat.” “That’s particularly appealing to lone wolves. We’ll see more of that in the U.S.”

Guns have become a tool for more U.S. terrorist attacks because the targets have shifted from property to people, Miller, the Global Terrorism Database manager, said. In the 1970s, there were routinely more than 100 terrorist attacks each year, while in some recent years, there have been fewer than 10 terrorist attacks — while the number of deaths hasn’t declined by nearly as much. Miller said the prevailing wisdom about terrorists is that they have changed from wanting to be watched, not to kill, to wanting both to be watched and to kill.

UPDATE (July 13, 10:55 a.m.): This article has been updated to reflect a new casualty count from law enforcement officials, who said on Monday that Mateen killed 49 people. The 50th fatality was Mateen himself, who was killed by the police.