A former assistant U.S. attorney testified Tuesday that a “lack of preparedness” on the part of Chicago Police Officer Robert Rialmo led to the officer fatally shooting Quintonio LeGrier and Bettie Jones in December 2015.

A short time later, though, a nationally known use-of-force expert doubled down on testimony he gave last year, saying that Rialmo had no choice but to open fire and that any reasonable officer in his position would have done the same — despite the shooting death of Jones, an innocent bystander who opened the door for police.

“Because a bad result occurs, [it] doesn’t mean someone did something wrong,” said use-of-force and firearms expert Emanuel Kapelsohn. “The person who did something wrong here was Quintonio LeGrier.”

Kapelsohn — who testified in a civil trial last year that was brought on by the lawsuit filed against the city and Rialmo by LeGrier’s estate — said that Rialmo demonstrated above average marksmanship when he shot LeGrier in December 2015.

“It’s very good shooting, [but] it’s tragic it turned out this way,” he said.

The testimony came during the second day of evidentiary hearings held by the Chicago Police Board. The testimony presented this week will be used by the full board to determine Rialmo’s future with the Police Department, though a decision isn’t expected for several months.

In the early hours of Dec. 26, 2015, Rialmo and his partner, Anthony LaPalermo, responded to calls of a domestic disturbance at a two-flat at 4710 W. Erie St.

Jones, the LeGriers’ downstairs neighbor, opened the door for the officers and told them the commotion was coming from the unit above hers. LeGrier’s father had barricaded himself in his bedroom while his son was trying to get in with an aluminum baseball bat.

Within moments of the officers’ arrival, LeGrier came barreling down the stairs and swung the bat at Rialmo as the officer stood on the front porch. Rialmo backpedaled and fired seven or eight shots, killing both LeGrier and Jones, 55.

It was the first fatal shooting by a CPD officer after the release of the Laquan McDonald shooting video.

Michael Gennaco, a former assistant U.S. attorney retained by the city, said Tuesday that Rialmo and his partner had 15 minutes — the time it took for them to drive from the Harrison District police station to the LeGrier home — to gather more information and formulate a plan about what they should do.

“They were forced to improvise and react, which is the worst situation an officer can be in,” Gennaco said. “It shows a lack of preparedness.”

Rialmo’s reaction to the shooting has been the focus of testimony several times during the first two days of hearings. After he shot LeGrier and Jones, he walked across Erie, where he told another officer that he “f----- up.”

Kapelsohn said the statement showed Rialmo has a conscience, though Gennaco said “it shows he realized he made a mistake within seconds.”

Janet Cooksey, LeGrier’s mother, was present again in court Tuesday, though — as she did the day before — she walked out of the hearing as it was in progress.

Though the line of questioning was barred from the civil trial last summer, attorneys for Rialmo raised the idea that LeGrier — who was in the midst of a mental health crisis and himself called police several times — wanted to be shot. They noted that he was not taking his prescribed schizophrenia medication at the time of the shooting.

Cooksey walked out as that testimony progressed, loudly saying: “Same person who called 911 three times.”

LaPalermo and one of Jones’ daughters, as well as several character witnesses, are expected to testify Wednesday. The hearings are set to conclude Thursday afternoon.