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Karen Read’s defense has called the expert believed to be its final witness as her trial in the death of Boston cop John O’Keefe, which began back in April, nears a close.

Prosecutors allege that Read, after a night out drinking, rammed O’Keefe in reverse with her Lexus and left him to die outside a house party during a record-setting blizzard on Jan. 29, 2022.

Dr. Andrew Rentschler testified briefly Tuesday afternoon and is expected to return to the stand for at least half a day Wednesday before the defense rests.

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He is the second witness from the crash reconstruction firm ARCCA, which now works for the defense but had been retained by the federal government ahead of Read’s first trial, which ended with a deadlocked jury.

Earlier in the retrial, his colleague Dr. Daniel Wolfe testified that damage to both O’Keefe’s clothing and Read’s SUV were inconsistent with a collision between a 2021 Lexus LX 570 and a pedestrian.

Rentschler is a biomechanist, an expert on how the forces of physics and engineering apply to the human body.

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Andrew Rentschler testifies

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“A mechanical engineer, for instance, might look at a piece of steel and the dimensions and size of the steel. If you apply a force in a certain direction with a certain magnitude, that steel will bend, and eventually it’ll break,” he explained. 

“As a biomechanist, biomechanical engineer, I do the same thing, but I look at the human body. How much force has to be applied, in what direction, to get a specific type of injury.”

Rentschler testified that a 15 mph collision between a crash dummy’s head and a taillight like Read’s wouldn’t have resulted in a skull fracture. Experts for both sides have previously testified that his fatal injury appears consistent with a backward fall – although the defense forensic pathologist Dr. Elizabeth Laposata said in her opinion, he landed on something “rigid” with a grainy texture – not the front lawn where he was found.

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Officer John O’Keefe poses for his official headshot

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Rentschler joined Laposata and another defense witness, Dr. Marie Russell, in the opinion that scratches and cuts on O’Keefe’s arm were not caused by an impact with the taillight. The defense theory is that they were inflicted by a dog bite.

Read told reporters Tuesday that she would not be testifying in her own defense – and that Renstchler is the last witness her team plans to call.

“It’s the right move,” said Grace Edwards, a Massachusetts trial attorney who is closely following the case. “It is the sole decision of the defendant whether to take the stand. In this case, Karen would have to withstand a rigorous cross-examination by Brennan, which would include clips from the documentary and any other evidence they have that we don’t know about from other statements she made.”

She would also have to face questions about her drinking, Edwards said, which could break the “upward momentum” of her defense as expert witnesses have repeatedly contradicted prosecutors’ claims. 

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