DEDHAM, Mass. — A prosecution witness played video of himself wearing a replica of John O’Keefe’s outfit while getting struck — albeit at slow speed — by the same model as Karen Read’s Lexus before testifying that he believes a strike occurred.

Read, 45, of Mansfield, faces charges of second-degree murder, manslaughter while operating a motor vehicle under the influence, and leaving the scene of a collision causing death. She’s accused of purposely backing her vehicle into Boston Police Officer O’Keefe, her boyfriend, and leaving him to die on a Canton front lawn.

Judson Welcher, a Ph.D. biomechanical engineer and accident reconstructionist, was the only witness called on Tuesday, the 21st day of Read’s second murder trial.

After hours of testimony regarding his methods of analysis, prosecutor Hank Brennan asked him for his conclusion: Did Read’s Lexus LX570 strike O’Keefe at around 12:32 a.m. on Jan. 29, 2022, outside 34 Fairview Road in Canton?

“Yes, based on the totality of the evidence, DNA, everything I’ve talked about, that is consistent with that happening and within a reasonable degree of scientific certainty that is what happened,” Welcher said.

The line led to an immediate objection by defense attorney Robert Alessi and became the last bit of testimony for the day — Judge Beverly J. Cannone dismissed the jury so that Alessi and Brennan could argue whether the line was admissible.

Admissibility

Alessi argued that for the witness to opine on whether a collision occurred overturns many years of precedent and that it’s answering “the ultimate question for the jury” and that the question “is never permissible in a trial.”

Alessi said that an analogous scenario would be a witness opining that a defendant stabbed a victim multiple times instead of remaining within their lane of expertise and saying whether injuries are “consistent” with such an occurrence.

In this case, Alessi argued, that would be for Welcher to limit his testimony to “two questions that a biomechanist always answers in these situations: Is the damage to the vehicle consistent with a pedestrian hitting a vehicle and … are the injuries to the individual consistent with being hit by a motor vehicle.”

Prosecutor Brennan countered that the defense repeatedly, including in the opening statement, opines in front of the jury that no collision occurred and that “they have no jurisprudence to say what we are limited to.”

“We don’t have to play peekaboo and close our eyes to the reality of the fact that the evidence supports the conclusion to a reasonable degree of engineering certainty that the defendant struck Mr. O’Keefe with her Lexus in front of 34 Fairview Road,” Brennan said.

Judge Cannone said that she would issue her decision Wednesday morning before trial. Testimony is scheduled to begin at 10 a.m.

The witness

Welcher is the second witness the prosecution has called from a firm called Aperture. The first witness was on the stand for two days and was subject to a whirlwind of cross-examination regarding “academic dishonesty.”

That witness, Shanon Burgess, had a bachelor’s degree written on his résumé, his bio on the Aperture website and his LinkedIn account but admitted on the stand that he had never earned the degree. A check on Tuesday confirmed that his LinkedIn profile had been deleted and his bio page had been updated to only display his associate’s degree.

Both Burgess and Welcher, a PhD who would be harder to impeach on qualifications, came to the same conclusion: that Read’s Lexus “techstream” system — which is usually used for maintenance diagnostics, Welcher said — was triggered by a “backup” event at around 12:32 a.m.

He testified that the data “window,” which doesn’t necessarily record the whole event, showed the vehicle pull forward 34 feet then reverse a total of 87 feet. At the end of that window, the vehicle was traveling at 23 mph in reverse, he said, and was still at 3/4 throttle, which he said suggested it traveled further than recorded.

Welcher not only examined the digital data but also did reconstruction of what the data suggested to come to his further conclusions. Several hours of the trial day had him going back and forth between a spreadsheet of the raw data and a PowerPoint presentation of how he reconciled that data — like coming to a real, human-recognizable time from the “running clocks” of the electronics.

But perhaps the most dramatic aspect of his reconstruction work came from video clips in his presentation where Welcher donned a replica of the outfit O’Keefe was wearing at the time of his death and had a driver strike him with an exemplar vehicle — another 2021 Lexus LX570 SUV — although at 2 mph instead of the 24 mph the prosecution asserts.

“Even a 2 mph impact redirects me back and causes me to rotate and take a step,” Welcher said following a clip of that happening.

Welcher, roughly O’Keefe’s size within an inch of height and four pounds heavier, also put fresh paint on the taillight area to see where on his right arm the vehicle may strike it and concluded it’s the same area as lacerations to O’Keefe’s arm. This is likely to be heavily cross-examined, as the defense has experts on their list to say those injuries came from a dog.

Finally, Welcher said he also examined the video of when Read’s Lexus tapped O’Keefe’s Chevrolet Tahoe while backing out of his garage at around 5 a.m. the morning of his death and concluded it could not have been the cause of Read’s busted taillight.

New witnesses

On Monday, the defense added four new names to their potential witness list, “for the sole purpose of authenticating records”: Matthew Coleman, of Quincy; Michael P. Arico, of Canton; Nicholas David White, of Milton; and Jonathan Diamandis, of Milton.

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