Mount Kisco, New York (CNN) -- No one disputes Douglas Kennedy carried his newborn Bo through the maternity ward of a suburban New York hospital and tried to take the baby outside. Nor is there any question an altercation ensued, involving nurses attempting to keep the baby on the floor.

But beyond that, there are only open questions -- especially about why it all happened and who is to blame.

Answers finally started to trickle in on Monday, as the trial of the late Robert F. Kennedy's youngest son began with opening statements and four witnesses taking the stand.

Judge John Donahue -- not a jury -- is hearing the case involving misdemeanor charges of harassment and child endangerment.

Kicking off the trial, the two sides outlined significantly divergent scenarios of what happened last January at Northern Westchester Hospital.

According to Assistant District Attorney Amy Puerto, the nurses were simply following protocol set up in newborns' interests when they tried to keep baby Bo from leaving the hospital.

The infant's father not only didn't back down, but he fought back -- even twisting one nurse's arm and kicking another -- the prosecutor said.

One of the nurses, Anna Lane, testified Monday that she'd initially reached in to hold open an elevator door to prevent Kennedy from leaving that way. When he headed toward a stairway door, she ran to it and put her hand on the doorknob.

"I had my hand behind my back and he grabbed my left hand -- which was on the doorknob -- and twisted my arm," said Lane.

Kennedy raised his foot and kicked another nurse, Cari Luciano, who'd come to help before entering the stairway, Lane added. But he only got down as far as the next landing, where he was met by security.

The defense team was tough on the nurse. Suggesting she had an ulterior motive given Kennedy's high profile, lawyer Robert Gottlieb questioned a tearful Lane why -- if recounting her story was so emotionally difficult -- she had recounted her version of what happened so soon after the incident on NBC's "Today Show."

Fellow defense lawyer Celia Gordon, in her opening statement, suggested Kennedy was calm and that the situation only deteriorated because of Lane. Any contact with the nurses was due to a father's instinctual reflex to "keep someone from ripping his baby from his arms," the attorney added.

"(Lane) was abrasive, confrontational and changed the entire tone of the discussion ... and wanted to let him know who was in charge of his baby," Gordon said.

The incident occurred at the hospital in Mount Kisco, located about 40 miles north of New York City and where this week's trial is taking place.

After Kennedy was arraigned in February, his attorney released a statement stating that nurses initially agreed to let him take the baby outside and demanding "appropriate disciplinary action" be taken against Lane and Luciano.

Timothy Haydock, a doctor at the hospital and longtime family friend who was with Kennedy at the time, asserted in the statement that "nurses were the only aggressors."

But neither nurse recognized Kennedy as the child's father and intended to protect the baby, who apparently had not been discharged, according to Elliot Taub, the attorney for both nurses. They were trying to enforce hospital rules, he said.

The hospital, in New York's Westchester County, issued its own statement in February supporting its nurses.

"At Northern Westchester Hospital, patient safety is our priority and we completely support the actions of our nursing staff in this case as they were clearly acting out of concern for the safety of a newborn baby."