This past week, the Department of Homeland Security held a news conference to clear up a few things about the federal paramilitary police force grabbing protesters on the streets of Portland, Ore. If the goal was to reassure everyone that these armor-clad agents were acting lawfully, it did not go well. Instead the conference revealed, with painstaking clarity, a very big problem: The deputy director of President Trump’s new federal police force does not know what the word “arrest” means.

This isn’t just semantics. In our legal system, the definition of the word “arrest” is critical because it marks an important dividing line under the Fourth Amendment. For an arrest to be legal, it must be supported by probable cause. That means the arresting officer must be able to point to specific facts that would make a reasonable person think that the person being arrested committed a specific criminal offense. By contrast, if the police have a noncoercive, consensual interaction with a civilian (sometimes called a “contact” or an “engagement” in law enforcement lingo), then the person has not been “seized” for Fourth Amendment purposes, and the police do not need to explain or justify why they approached the suspect in the first place.

In other words, you can think of the word “arrest” as an on-off switch for the Fourth Amendment’s essential protections. When the police arrest someone, they are constrained by the Constitution. Before then, the Constitution’s protections are substantially weaker — if they exist at all.

Given the central importance of the word “arrest” in the constitutional law of policing, it is chilling to see a commanding officer of a law enforcement agency demonstrate a basic misunderstanding of its meaning. But that is what happened during the administration’s news conference. When asked whether federal agents in Portland are complying with the requirement to have probable cause for arrest, Richard Cline, deputy director of the Federal Protective Service, took the microphone. “You’re probably talking about the van,” he replied. Cline was referring to a viral video that shows two camouflaged federal agents grabbing an apparently peaceful protester off the street and taking him away in an unmarked van for questioning.

In terms of probable cause, Cline candidly acknowledged that the agents in the video “didn’t have what they need” to conduct an arrest. They released the man when lawyers from the Department of Justice told them that there was no lawful basis to continue detaining him.

Those lawyers were right. The Supreme Court has made clear that officers cannot arrest people simply for standing next to someone who may have committed a crime. Rather, police must have probable cause that is “particularized with respect to” the person they want to arrest. Yet, as Cline acknowledges, the agents never had any reason to think that the man in the video — as opposed to others in the crowd — had done anything wrong.

In other words, they did not have probable cause. Not when they spoke to the lawyers. Not when they put the man in the van. Not ever.

Still, the government maintained that what the agents did was lawful, and Cline’s explanation for that belief is the heart of the problem. It all comes back to that crucial word, “arrest.” DHS says pulling a man off the street and carrying him away in an unmarked van was just a “simple engagement.” “It was not a custodial arrest,” Cline said. In other words, the officers supposedly complied with the Fourth Amendment because, in Cline’s view, they didn’t need probable cause: There was no arrest, he says.

This assertion is glaringly wrong — and it has been for 40 years, since the Supreme Court’s 1979 opinion in Dunaway v. New YorkIn that case, the justices had to decide whether police officers violated the Fourth Amendment when, without probable cause, they took someone “into custody, transported him to the police station, and detained him there for interrogation.” Their answer, reaffirmed over the years, was unequivocal: Such conduct is “indistinguishable from a traditional arrest,” which need not be accompanied by bureaucratic processes like fingerprinting, booking or a formal arrest record to count as an arrest for purposes of the Fourth Amendment.

So, according to the Supreme Court, the federal agents in that video really did arrest the man they put in the van —  without probable cause, by the government’s own admission. That means they violated the Fourth Amendment.

Crucially, what happened in that video is not an isolated incident. A man named Mark Pettibone describes a similar event in which, according to his account in The Washington Post, “men in green military fatigues” put him in “an unmarked minivan,” took him “to the federal courthouse and placed him in a holding cell” for questioning. Conner O’Shea, who was with Pettibone at the time, recorded audio of the incident and says a second unmarked van pursued him as well. Beyond these incidents, there are also disturbing videos of federal agents using excessive force against a peaceful Navy veteran and, on a separate occasion, shooting a young man in the face with a nonlethal bullet that fractured the man’s skull.

All of these episodes raise serious concerns that the newly beefed up federal paramilitary police force deployed to the streets of Portland is breaking the law, with dire consequences. Arresting people is a large part of what police officers do, and the impact of an arrest can last a lifetime. It is thus absolutely essential that arrests be lawful — that they be supported by probable cause and not be executed via excessive force, as the Constitution demands.

The government, however, cannot lawfully exercise this awesome power of arrest if it doesn’t realize it is, in fact, arresting people in the first place — in Portland, or wherever the president sends these officers next.

Twitter: @AndrewMCrespo