FORT LAUDERDALE, Fla. – Prosecutors spent hours Tuesday trying to prove the Florida school shooter’s case Nicholas Cruz deliberately failed tests to see if he was suffering from fetal alcohol spectrum disorder, the main reason his lawyers say he killed 17 people four years ago.

But after presenting dozens of charts of IQ and other exam results and lengthy explanations of averages and standard deviations that even prompted the judge to joke that she understood why some jurors drink strong Cuban coffee, Assistant Prosecutor Jeff Marcus pulled his trump card.

He turned to the simplest test given to Cruz: How fast can a convicted murderer tap his dominant left index finger?

In tests conducted earlier this year by experts hired by his attorneys, Cruz averaged 22 taps in 10 seconds. Robert Denny, a neuropsychologist for the prosecutor’s office, said the average male score on this test is 51, and a score of 22 is just a person with a severe brain injury that causes physical stiffness.

Marcus then played a clip of the cell phone video February 14, 2018massacre at Marjory Stoneman Douglas High School in Parkland, where Cruz was recorded shooting with AR-15 type semi-automatic rifle 20 times in seven seconds with a pause of one second – each shot required a separate trigger pull. He then showed security video of Cruz smoothly turning and firing two shots at a coach who tried to stop him, and another of him quickly removing a magazine from his gun and loading a new one.

A person who really did that badly on the finger tap test “wouldn’t be able to pull the trigger like that,” Danny said, and he also wouldn’t be able to shoot and reload in one motion like he did.

Cruz, 24, pleaded guilty a year ago before the murder of 14 students and three employees — the court will only decide whether he is sentenced to death or life without parole. Cruz needs a unanimous vote from the seven-man, five-woman jury to receive the death penalty. Anything less and his sentence would be life.

Chief prosecutor Mike Satz hopes Danny’s testimony will disprove the defense’s claim that abundant drinking Cruz’s birth mother, during pregnancy, set him on the path of a lifetime whimsical and sometimes violent behavior which ended in shootings. The defense also tried to show that his adoptive mother, Linda Cruz, became overcrowded after her husband died when Cruz was 5 years old and she never received full treatment for his mental health issues. She died less than three months before the shootings.

Denny, a contract worker in the federal prison system, testified Tuesday that he disagreed with defense experts’ contention that Cruz suffered from fetal alcohol disorder. The Missouri practitioner said the former Stoneman Douglas student’s IQ of around 90 and the results of tests he underwent after the attack did not support that diagnosis.

“Mr. Cruz’s scores are exactly what we would expect from someone who doesn’t have any fetal alcohol issues,” Danny said.

Denny will be questioned by Cruz’s lawyers on Thursday. Court will be closed on Wednesday due to the Yom Kippur holiday. The prosecution is expected to complete its rebuttal in the coming days, with closing arguments and arguments scheduled for next week.