Many are now asking the same question.
Johnson, a black student at the University of Virginia, suffered injuries early Wednesday when he was arrested by officers with the Department of Alcoholic Beverage Control (ABC) in Charlottesville.
As he's pinned, video captures him yelling: "I go to UVA! ... You f*****g racists! What the f**k?"
An agent can be heard telling the student to stop fighting.
"I trust that the scars on my face and head will one day heal, but the trauma from what the ABC officers did yesterday will stay with me forever," Johnson said in a statement read by his attorney Thursday.
He said his "head lay bloody, but unbowed," and urged calm.
"I believe we as a community are better than this," he said. "We cannot allow the actions of a few officers to ruin the community of trust we have worked so hard to build."
The video does not show the moments leading up to the arrest. But many say it's the latest example of excessive force by officers against a black man.
The arrest warrant for Johnson describes the student as "agitated and belligerent." He was allegedly detained for public intoxication and obstruction of justice.
Denied entry at a bar?
The incident started when Johnson, 20, was turned away from a bar during St. Patrick's Day celebrations. He is under the legal drinking age.
Virginia ABC agents approached him and decided to "further detain the individual based on their observations and further questioning," the agency said.
"In the course of an arrest being made, the arrested individual sustained injuries."
Johnson's attorney gave a more detailed, and slightly conflicting, account:
He said his client was standing on a sidewalk when an employee of the bar approached him, and asked for identification. Johnson presented his valid ID card issued in 2011. The employee then asked for his ZIP code, and Johnson recited the ZIP code of his mother's current address, rather than the ZIP code on the ID, his attorney said.
Nevertheless, agents questioned Johnson about being in possession of a fake ID, Daniel Watkins told reporters.
"Just before handcuffing him, police took Martese to the ground, striking his head on the pavement and causing him to bleed profusely from the gash on his head," the attorney said. He said Johnson's head wound required 10 stitches.
Student Jennifer Goldman didn't see what started the incident, but she and her boyfriend were passing as officers held Johnson down, she said, adding that she recognized Johnson from last year's orientation as a "nice and funny" guy.
"He was definitely upset. I don't know how belligerent he was because I didn't see the arrest or takedown, but he was definitely upset," she said.
As for the officers, she said, she didn't see them do anything violent, "but they were definitely restraining him and they weren't really doing much to take care of his head, but they were just holding him down."
Students rally for Johnson
Hundreds of University of Virginia students of different races protested Wednesday night.
"For his face to literally be bashed in by concrete -- there's no words to describe that," a protester told CNN affiliate WTVR
Johnson himself took the microphone, calling for peace.
"Regardless of your personal opinions and the way you feel about subjects ... please respect everyone here," he said. "We are one community. We deserve to respect each other, especially in times like this."
Governor demands investigation
Gov. Terry McAuliffe has called for an independent investigation by Virginia State Police to see whether the alcohol control agents used excessive force. Both an administrative review and a criminal investigation are underway.
"Both elements of this investigation will take time to conduct and complete," Virginia State Police Superintendent Col. W. Steven Flaherty said. "We owe it to both Mr. Johnson and the Virginia ABC to be painstakingly thorough in determining the facts of the situation through interviews, evidence collection and analysis, and investigative procedure."
UVA President Teresa Sullivan said she was disturbed by images of the arrest and contacted the governor's office to ask for the investigation.
"I felt it in my stomach," she told WTVR. "Just seeing the blood run down that young man's face, I wanted to know what happened."
The university's Black Student Alliance issued a statement denouncing the incident.
"Today, we are reminded of the gruesome reality that we are not immune to injustice; as University students, we are not impervious to the brutality that has reeled on news cycles around the country," the group said Wednesday.
"We have marched and shouted that we are Trayvon Martin
and Michael Brown, but the proximity of this morning's brutality to a member of our community has deepened that wound."
The special agents involved in the arrest will be restricted to administrative duties, pending the results of the investigation, the Virginia ABC said.
Student leader 'devastated'
Johnson is a third-year student double-majoring in Italian and media studies. He has no criminal record, according to his attorney, and holds a variety of leadership positions on campus.
"As evidenced by both his academic and extracurricular achievements, Martese is a smart young man with a bright future," Watkins said. "He is absolutely devastated by yesterday's events. Currently, we are preparing to investigate and defend this matter vigorously."
Johnson will appear in court on March 26 for misdemeanor charges of obstruction of justice without force and public intoxication, CNN affiliate WWBT
reported. He told CNN that he was not intoxicated.
Marcus Martin, chief officer for diversity and equity at the university, said he'd spoke to bystanders who'd indicated that Johnson also did not resist arrest.
"This should have been deescalated," he said. "The officers could have taken him to a side, I believe, and just said: 'Okay, well, let's have a conversation,' rather than push him down to the pavement."
UVA's president urged anyone who witnessed the arrest to contact authorities.
"We have not yet clarified all of the details surrounding this event," Sullivan said, "but we are seeking to do so as quickly as possible."