The investigation by the Army Criminal Investigation Command grew out of a review the Army was already conducting of Johnson's entire service record -- a review that would be done as a matter of course, due to the high-profile nature of the attack on the Dallas police, the ongoing civilian investigation into the matter and the Army's own review of the record before making portions of it available to the public.
The Army "wants to have a full understanding of him, and his service," an administration official told CNN.
Authorities say Johnson shot and killed five police officers
in Dallas July 7 during a largely peaceful protest. Seven other officers and two civilians were wounded in the shooting. Johnson was killed by a police robot equipped with an explosive device after an hours-long standoff.
Johnson received an honorable discharge from the Army in April 2015, but the official said "some administrative errors in processing his record" may have led to him receiving that undeservedly.
Sexual harassment accusation in Afghanistan
In the process of reviewing Johnson's service record, the Army found some incomplete information regarding potential allegations of sexual misconduct against him, the official said.
Johnson was accused of sexual harassment by a female soldier in the spring of 2014 while they were both serving in Afghanistan, attorney Bradford Glendening told CNN.
Johnson returned to the United States and his reserve unit in July of 2014, and Glendening represented him in military proceedings here, the attorney said, characterizing it as "sort of a hostile work environment" case.
Because it was an administrative complaint and not a criminal case, there was no court martial involved.
The accuser made two requests: that Johnson get some mental help and that she be granted a protective order against him that would include herself, her family, her current and any future residence and her place of work and any place she may reside. Glendening said the protective order was granted.
The Army instituted an administrative proceeding in which Johnson would be dismissed with an "other than honorable discharge," Glendening said, but in September 2014 Johnson waived his right to a hearing and asked for the discharge to be upgraded.
In April 2015, the Army said, Johnson received an honorable discharge.
Was honorable discharge warranted?
But now Army investigators are reviewing the paperwork that was handed off to his reserve unit when he returned from Afghanistan to determine if it contained all the documentation that would have spelled out the initial sexual harassment allegation and possible other allegations of sexual misconduct.
The official said that contrary to initial reports, no paperwork has been found yet indicating that Johnson was to be offered anything other than an honorable discharge from the reserves, raising the question of why the reserve unit did not have full information on him. "There may have been some administrative errors in processing his record," that led to an honorable discharge, the official said.
The official would not detail the information now being reviewed, but typically, an allegation of sexual assault would automatically trigger a criminal investigation. "Some things might not have been as fully investigated as they should have been," the official said.
There is no indication in Johnson's record that he had any inclination toward committing murder, the official emphasized.
The Army is reviewing all of Johnson's records, including those regarding his training, medical records and deployment paperwork. In the medical record, officials are looking for any indications of diagnoses or treatment of any physical or mental health conditions that may be relevant to Johnson's actions.