(CNN) A police officer who claimed she killed a Dallas man in his own apartment in the mistaken belief that he was in her home was indicted Friday on a murder charge, authorities said.

The indictment of Amber Guyger comes more than two months after she was arrested in the shooting death of Botham Shem Jean at the Dallas apartment complex where both lived -- a killing that sparked days of protests.

Guyger was arrested after the September shooting and charged with manslaughter by the Texas Rangers, the lead investigative agency, Dallas County District Attorney Faith Johnson said at a news conference.

When asked why the grand jury indicted Guyger on the more serious offense of murder, Johnson replied, "We presented the evidence and we explained the law."

Johnson said murder constitutes someone "intentionally and knowingly" committing a crime, whereas manslaughter involves "recklessly doing something."

"At the moment of the shooting it was a knowing ... offense," Johnson said.

The court records Friday showed both a manslaughter and murder charge entered in Guyger's file, but a clerk of court clerk confirmed that the murder charge is the one prosecutors are moving forward on.

Robert Rogers, Guyger's attorney, was disappointed but not surprised by the indictment, given what he called an "outpouring of vindictive emotion" in a statement late Friday.

"This is a terrible tragedy that resulted from a true mistake," he said. "We are confident that a dispassionate jury in a fair forum will objectively apply the law to the facts and find Amber not guilty."

Guyger had already been arrested again and released on bond, Johnson said at the news conference.

Dallas Police Chief U. Reneè Hall said in a statement everyone in the department "continues to feel anguish about this difficult and tragic event."

"We recognize and understand the national discord regarding the relationship between law enforcement and the communities we serve," Hall said.

She also added that the department has "developed a framework for policy change" by restructuring the Citizen Review Board, revamping implicit bias training and seeking input from employee advisory and community advisory boards.

"We have more work to do and we remain committed to improving our relationships throughout the city," Hall said.

The indictment is a "step toward justice" for Jean's family, said Sharon Watkins Jones, ACLU of Texas Director of Political Strategies.

"The Jean family's loss cannot be restored to them, but we will continue to work to ensure that police officers are held to the same standards as everyone else in the Dallas community and across the state of Texas," she said.

'He didn't deserve it," Jean's mother says

Botham Jean's mother, Allison Jean, said during a news conference that she's satisfied with the murder indictment.

Guyger "inflicted tremendous evil on my son," Allison Jean said. "He didn't deserve it. He was seated in his own apartment. He felt safe and he was violated by her coming in and murdering him," she said.

Allison said she hopes a proper penalty will cause Guyger to reflect on what she has allegedly done and the pain she has caused.

Family attorney S. Lee Merritt said the grand jury decision is "groundbreaking, but it is also just a start."

Officer thought she was in her own apartment, warrant says

Guyger, who is white, was off-duty when she encountered Jean , an 26-year-old unarmed black man, in his apartment on September 6, police said. Still in her uniform, Guyger parked her car in the complex and walked to what she believed was her apartment, according to an arrest warrant affidavit

The door was slightly ajar as she tried to use her key, which has an electronic chip. When she opened the door, she saw the interior was almost completely dark, according to the affidavit. She described seeing a large silhouette and, believing there was an intruder in her apartment, drew her firearm.

Amber Guyger is charged with killing Botham Shem Jean on September 6.

She issued verbal commands, but Jean, being in his own home, did not heed them, and Guyger fired two shots, hitting him once in the torso, the affidavit said.

Guyger, a four-year veteran, then entered the apartment, called 911 and started administering first aid to Jean. She turned on the lights while on the phone with 911, and only when asked for her address did she realize she was in the wrong apartment, she told police.

Jean died at a hospital. Guyger was arrested September 9 on suspicion on manslaughter, and was released from the Kaufman County Jail after posting a $300,000 bond.

The Dallas Police Department fired Guyger during a hearing September 24, the police chief said.

Johnson will not prosecute the case. She was defeated in the November election by John Creuzo t, who will take office in January.

JUST WATCHED CNN was granted access to Botham Jean's apartment Replay More Videos ... MUST WATCH CNN was granted access to Botham Jean's apartment 03:30

The shooting sparked days of protest . Police deployed pepper balls on demonstrators a week after the shooting. Protesters angry with the lack of public information in the case interrupted a City Council meeting to demand accountability and more police oversight in general.

Jean's parents filed a lawsuit in federal court against Guyger and the city last month, alleging Guyger used excessive force.

Correction: An earlier version of this article said that Guyger was indicted on a charge of manslaughter. The officer has been indicted on a murder charge. The headline and article have been updated to reflect this change.