Six weeks ago, a gunman fired more than 30 shots on a New York City underground train, wounding 10 people.
Ten days ago, another gunman in the subway killed a passenger in what officials said appeared to be a random attack.
Following the attacks, New York City's mayor, or leader, suggested a high-tech idea: deploy security equipment at subway entrances that can sense the presence of guns.
Such machines exist. They can be found at large sports centers, airports and many other places where crowds gather.
But security experts say that the use of such technology in New York's huge subway system would be difficult, if not impossible.
The machines, called scanners, only provide information. Human security agents would be required at each site to act on the information.
"You're going to have to tie up a lot of officers doing this," said James Dooley. He is a retired New York Police Department captain who served in the department's transit division. "We have hundreds of stations, and the fact of the matter is that putting someone at every entrance to every station is logistically impossible," he added.
New York City Mayor Eric Adams is a former police captain. He said he understands the difficulties of such a plan. But he has said it might still be worth trying at a few subway areas as a prevention measure.
"We want to be able to just pop up at a station some place so people don't know it's there," Adams said, "similar to what we do when we do car checkpoints."
Adams spoke about the weapon-screening technology after the subway attacks in his city. Mass shootings in Buffalo, New York, and in Uvalde, Texas, intensified the debate over how to deal with gun violence.
Evolv, a Boston-area company, is a large provider of the technology. It has systems in place at several large sites in Atlanta, Georgia and Nashville, Tennessee. Evolv recently set up the machines at Lincoln Center in New York City. The company has not worked in any mass transportation systems.
Evolv says its machines can screen 3,600 people in an hour. However, the technology also makes mistakes, sometimes identifying non-threatening objects as weapons.
Similar screening devices are made by Thruvision, an England-based company. Those devices were part of an early test program in the Los Angeles mass transit system in 2018. They are currently used when threat levels are raised, said Los Angeles Metro spokesperson Dave Sotero. The machines project scanning waves at people passing by from a distance.
Identifying someone with a weapon is only half the problem.
"It's also manpower," said Donell Harvin, a senior policy researcher at the Rand Corporation and a former security chief for the Washington, D.C. government.
Mayor Adams has not publicly discussed how much the machines, and operating them, could cost New York City. But Harvin said the price could be very high.
I'm Ashley Thompson.