Gabe Rottman is the Washington director for PEN America.

If I'm peacefully marching and a person beside me throws a brick, who broke the law? In Washington, D.C., the capital of a nation of growing protest, the answer to that question should be relatively easy. Unfortunately, it's not.

In fact, this question has bedeviled the city for 50 years. Dangerous ambiguities in our riot laws — most notably the D.C. riot act — threaten to tar peaceful protesters as criminals (and permit authorities to charge protesters with crimes carrying draconian penalties vastly disproportionate to any alleged harm). As a result, 59 protesters arrested during last year's inauguration — including at least one journalist — still face years in jail.

The D.C. riot act criminalizes an assembly of five or more people involved in "tumultuous and violent conduct" that poses a "grave" danger of injury or property damage. But the law provides no guidance on what "tumultuous and violent" or "grave" mean. This ambiguity lets prosecutors aggressively pursue protesters with felony charges — protesters who had absolutely no intention of doing anything other than exercising their First Amendment right to assemble.

The riot act dates to the widespread unrest over civil rights during the late 1960s, and its troubling ambiguities to a court case that arose during the 1968 protests following the assassination of the Rev. Martin Luther King Jr. On April 5 of that year, looters took advantage of the mayhem to rob a liquor store near what is now Barracks Row. Police then arrested a man who claimed he was out looking for his wife, noticed a bag of liquor from the store on the ground and picked it up.

In United States v. Matthews, the U.S. Court of Appeals for the District of Columbia Circuit ultimately found that simply picking up the looted liquor was enough to label the defendant a rioter (versus just a thief). In the decision, the majority of Judges Carl McGowan and Edward Tamm (Judge J. Skelly Wright dissented) wrote that it "requires no uniquely sharpened social perceptions to know what one should — and what one should not — do when confronted with the incendiary conditions in the streets."

In other words, courts and cops know a rioter when they see one. That same theory is operative today.

On Jan. 20, 2017, a large group of "Disrupt J20" protesters marched through downtown Washington. A handful of individuals who authorities allege were part of the demonstration caused more than $100,000 worth of damage. The police arrested everyone they could by "kettling," or trapping in a confined space, and prosecutors charged most of those corralled with felony rioting.

Judge Lynn Leibovitz, who presided over the first J20 trial, cited Matthews in refusing to drop the primary riot charges against the defendants. Even if the J20 defendants did not themselves join in any violence or property damage, she held, the riot act could still permit a juror to find them just as guilty of rioting as those who did.

Thankfully, a jury of Washingtonians acquitted the first six of the protesters to face trial, finding, as one juror explained, that the prosecution had failed to prove anything other than "these six individuals walked in a protest." Last month, federal officials dropped charges against 129 defendants. But these developments only highlight the indiscriminate nature of the arrests and prosecutions of the protesters, 59 of whom still face years in jail.

Accordingly, much more must happen to protect freedom of assembly in our nation's capital.

As Wright explained in his Matthews dissent, the riot act "does not include the common law requirement of a common purpose or intent on the part of the rioters." We should amend the law to require the proof of intent that Wright was looking for and couldn't find back in the 1960s.

Given these legal infirmities, the Justice Department should also drop all charges against the remaining J20 defendants, unless prosecutors really have evidence putting a brick in a specific defendant's hand. (And, even then, the punishment must fit the crime; a decade in jail does not.)

Finally, we should strengthen the District's First Amendment Rights and Police Standards Act, passed after improper mass arrests in the early 2000s. The law is intended to limit tactics like kettling and to require dispersal orders in most cases before the police can disrupt an assembly. According to police officials themselves, those and other prophylactic rules weren't followed in the J20 case. It's up to the D.C. government to improve the law and hold to account those who violate it.

As President Trump's second year in office begins, it's clear this will not be an age free of protest. Sometimes that protest will be ardent. The First Amendment exists because political dissent, from the left or right, is patriotic and must be given room to thrive. That means ensuring that the authorities can distinguish between crime and speech. As it stands now, D.C. law is unable to do so.