They argue the knife was illegal and, as a result, Gray's arrest was lawful.
Baltimore City State's Attorney Marilyn Mosby has said the knife was legal under Maryland law, meaning that police had no reason to detain him.
The question of whether the knife was legal has raised a host of other questions. We ask and answer some of them here:
Court documents say it was a "spring-assisted, one-hand-operated knife." Mosby has said the knife was not a switchblade.
Switchblades are illegal in Maryland, but the law is even stricter in Baltimore.
"Baltimore City has a law that says it's not only illegal to have a switchblade, but it's also illegal to have a spring-action knife," said Andrew Alperstein, a defense attorney.
Again, Mosby has said the knife was legal. So has an attorney for the Gray family, calling the allegation a "sideshow." Gray was carrying a "pocket knife of legal size," according to attorney William Murphy.
But the police investigation found that the knife was illegal under Baltimore city code.
CNN has not been able to independently examine the knife in question.
Why does the type of knife matter?
In documents filed on behalf of Officers Garrett Miller and Edward Nero, attorneys argue that a "careful inspection of the knife recovered from Mr. Gray will reveal specific characteristics of the knife which will reveal that the knife was not lawful under Maryland law."
Miller and Nero have each been charged with two counts of second-degree assault, two counts of misconduct in office and one count of false imprisonment.
The charges stem from the fact that Mosby believes Gray's arrest was unlawful, based on the type of knife he had.
If the arrest was, in fact, lawful -- meaning that Gray's knife was illegal -- then Miller and Nero did not do anything wrong, according to documents filed by their attorneys.
Let's assume the knife was legal: Should the officers be charged?
When officers make mistakes, the usual consequence is the exclusion of evidence, Alperstein said.
"Now in this case the prosecutor's gone further, much further. What she said is -- if you violate the law by arresting these people falsely, then we're going to charge you with assault for touching them, false imprisonment," he said.
"This is very troubling and the police are worried about it," said Alperstein, adding that the charges could have a chilling effect on police doing their job.
Former Baltimore Deputy State's Attorney Page Croyder agreed.
"It would be dangerous for the prosecutor to bring charges like this based on a mistake by the police officer," she said.
"You're setting a precedent that any police officer who arrests without probable cause can be not just civilly sued, but criminally charged. These are people who are not lawyers. Lawyers disagree about probable cause. Judges disagree. And as long as officers are acting in good faith ... to subject them to criminal charges is going to put a chill on the whole police department."
Now, let's assume the knife was illegal: Does it really matter?
Officers only found the knife after they chased and caught Gray, who police say fled unprovoked.
To many protesters, a focus on the knife seems irrelevant -- a way to justify the chase after the fact.
Some also say that whether or not Gray had a knife, and whether or not it was legal, does not matter when viewed in the larger context of what happened to him after his arrest.
To them, no suspect deserves the sort of treatment Gray was allegedly subjected to. They note the punishment for having an illegal knife is not death.