Nero faces assault, misconduct in office and reckless endangerment charges. The assault charge carries a maximum penalty of ten years in prison, while the other charges have five-year maximums.
Gray died last year, a week after his neck was broken in a police van. He was handcuffed and shackled in the van, but unrestrained by a seat belt. The department's policy calls for detainees to be belted in.
Prosecutors say Nero unlawfully arrested Gray, and was negligent when he failed to buckle the man in.
The judge asked Deputy State's Attorney Jan Bledsoe whether every police officer who makes an arrest without probable cause should be charged.
"You're saying that's a criminal assault, as opposed to just suppressing what's found in that search?" the judge said.
The prosecutor replied, "yes, it's a criminal assault."
Gray's death set off protests and riots over police officers' alleged mistreatment of blacks and led to a city-wide curfew as the National Guard came in and helped restore order.
Nero waived his right to a jury trial, instead choosing to leave his fate in the hands the judge. He did not testify.
Nero's attorney argues his client didn't arrest Gray, and that it is the police van driver's responsibility to buckle in detainees. The judge is set to deliver his verdict Monday.
Nero was on bicycle patrol in a high-crime area in the Western District on April 12, 2015, alongside Officer Garrett Miller and Lt. Brian Rice. While patrolling the area, Rice made eye contact with Gray, who took off running. Rice called over the radio for backup, and Nero and Miller joined the foot chase. Miller caught up with Gray and handcuffed him.
Miller, who also faces charges, said during testimony that he alone arrested Gray.
After Gray was in custody, officers found the knife. Prosecutors said the knife was legal and the defense said it was illegal.
Nero told investigators that he touched Gray only twice: once when he helped the man, who'd already been handcuffed, sit up and look for an inhaler. The second time was when the van made a stop two blocks from the arrest site, and two officers secured him in leg shackles. Nero helped those officers slide Gray onto the floor of the van, head-first.
Nero is the second officer to stand trial. Officer William Porter's manslaughter trial ended with a hung jury.