Edward Nero, left, a Baltimore police officer who was involved in Freddie Gray's arrest, exits the Baltimore City Circuit Court with his attorney, Marc Zayon, after the first day of his trial on May 12. (Mark Makela/Getty Images)

The state of Maryland rested its case Monday against Baltimore police officer Edward M. Nero, who is facing four misdemeanor charges, including assault and reckless endangerment, in connection with the arrest of Freddie Gray last year.

In the trial’s most anticipated moment, the prosecution called as its final witness one of the other officers charged in the case, Garrett E. Miller. It was the first time that one of the defendants has testified against a fellow officer also facing charges.

Miller and Nero were among the first police officers to come in contact with Gray as the 25-year-old was chased through his West Baltimore neighborhood on April 12, 2015. In his testimony Monday, Miller said Nero didn’t touch Gray during the initial arrest, although he did later help load Gray into a police wagon.

Nero is the second of six Baltimore officers to face trial in connection with the arrest, which ended with Gray unconscious in a police van where he had been placed with his hands cuffed behind his back and his legs shackled. Gray died a week later without regaining consciousness, sparking protests and rioting across the city.

Over the course of 2½ days, prosecutors called 14 witnesses in an effort to show that even though Gray ran from police, Nero did not have probable cause to arrest him and, thus, the arrest constituted assault. Prosecutors also argued that Nero later endangered Gray by not seat-belting him when he was placed in the police van.

Events leading to Freddie Gray’s death, explained in augmented reality

But Nero’s attorneys pushed back, saying that their client had acted reasonably and within the law. After the prosecution rested its case in front of a packed courtroom that included members of Gray’s family, defense attorney Marc Zayon requested an immediate acquittal by Judge Barry G. Williams, who will decide Nero’s fate because the officer opted for a bench trial rather than face a jury. The judge denied Zayon’s request.

Miller, who was on bike patrol with Nero that day, was forced to testify by prosecutors, with approval from the court. His testimony in Nero’s trial cannot be used against him when he faces trial on the same charges in July.

During nearly two hours of testimony, Miller told the court that he and Nero became involved when they heard orders from Lt. Brian W. Rice, who is also charged in the case, to begin a chase although neither officer knew why Rice had ordered it.

Miller said he and Gray were startled when they came face to face in an alley. The officer said he chased Gray, holding out his stun gun and shouting “Taser, Taser, Taser, get on the ground.”

When he and Nero caught up to Gray, he said, the suspect did not resist.

“When he heard Taser and saw the other officer, he just gave up,” Miller told police investigators in an interview he read from the witness stand.

Prosecutors pressed Miller about Nero’s involvement in Gray’s arrest, but Miller testified that he alone arrested and handcuffed Gray although Nero was standing nearby. Nero’s attorneys had said in opening statements that Nero didn’t touch Gray until he asked for his inhaler.

People pass a mural in Baltimore depicting Freddie Gray a year after the protests that were sparked by Gray's death in police custody. (Brendan Smialowski/AFP/Getty Images)

Courtroom observers differ as to whether the prosecution has been effective in laying out its case to convict Nero.

“Miller did more to help the defense today than the state,” said Warren S. Alperstein, a former Baltimore City prosecutor who is now a defense attorney and has been closely following the case. “He testified unequivocally that Officer Nero played absolutely no role in handcuffing or detaining Freddie Gray.”

Alperstein believes that defense attorneys will try to make the case that Nero acted in the same way that “any reasonable Baltimore Police Department officer would if they were in Nero’s shoes.”

But University of Maryland law professor Doug Colbert argued that the government’s case isn’t as weak as some critics have made it out to be.

“I think there’s a strong legal basis for the prosecution to make a strong argument for conviction,” Colbert said. “What’s at stake here is that police make many arrests without probable cause and this case allows the potential for reform of the police decision to arrest. You can’t just say that because you’re a black guy running away in a high-crime area that you’re a danger.”

During his testimony, Miller responded to questions about how Gray was transported. He acknowledged to lead prosecutor Michael Schatzow that he told police investigators on the day of the arrest that he thought Gray was put on the van’s bench.

Miller testified that the police van drove a block away and made a stop. There, he said, he and Rice pulled Gray from the van by his feet. Miller replaced the handcuffs that Gray was wearing with flex cuffs and placed Gray in leg irons.

When Nero and Rice attempted to put Gray back in the van, his body was limp, Miller said.

“He was acting like a dead fish?” Schatzow asked.

“Yes,” Miller answered.

Earlier in the day, Joseph McGowan, a bioengineering consultant, testified that Gray died from a “diving injury,” his body continuing to move forward after his head has hit a hard surface.

The injury was caused either when Gray was lying on the ground and the van stopped suddenly or when he rose to his feet, lost his balance and hit his head against the van wall, according to McGowan’s analysis.

He testified that it would not have occurred had Gray been restrained in a seat belt.

Williams asked McGowan whether Gray could have suffered his injuries after being seated on the bench, still handcuffed and with his legs shackled.

In his trial last fall, Officer William G. Porter testified that when he checked on Gray partway through the ride, he lifted Gray off the floor and placed him on the bench.

“I can’t rule out that he could get that injury,” McGowan said.

The ambiguity is important, because if Gray was injured after Porter moved him, his defense attorneys can argue that Nero’s role in placing him face down in the van is irrelevant.

Former Baltimore police commander Timothy Longo, who was the first witness called by the defense, said on the stand that Nero’s “conduct was objectively reasonable based on the circumstances with which he was confronted.”

But although Longo said that officers can use “discretion and good judgment” when it comes to any general order issued by the police commissioner, including an order to seat-belt all detainees, he agreed with Schatzow that it “would not be appropriate” to ignore those rules.