Two brothers accused of torturing and killing a homeless man found in a riverbed in Santee, California, pleaded not guilty to murder, torture and kidnapping charges in court Monday.

George Lowery, 50, was beaten with "fists and feet" in an assault that homicide investigators describe as "a very horrific event." He was hospitalized with head injuries after his wife found him unconscious April 24, face down under a piece of plywood near Chubb Lane and N. Magnolia Avenue. Lowery died five days later.

Brothers Preston Mostrong, 19, and Austin Mostrong, 20, are accused in the killing, facing criminal charges including robbery, kidnapping, torture and murder. They pleaded not guilty through their attorney Monday and denied all charges. The two were held on $3 million bond.

Lowery's family and other witnesses said the attack was in retaliation after Lowery tried to stop the brothers from bullying homeless people. Lowery and his wife lived in a homeless encampment in the area.

An altercation unfolded between Austin Mostrong and another resident of the river bottom on April 20, deputies said. Lowery did not fight with Austin Mostrong but may have been involved, according to investigators. He was found beaten four days later.

"We don’t believe this is a random act," said Lt. Kenneth Nelson, of the San Diego County Sheriff’s Department Homicide Detail. "We believe he was targeted."

Physical evidence at the scene and witness statements led investigators to the brothers, Nelson said.

No motive was given in court, but the prosecutor said both defendants admitted to taking part in the beating.

At the time of the arraignment, the Mostrong brothers were out on probation, one for theft and the other for misdemeanor assault.

Family members of the victim and those of the defendants were present for the emotionally charged hearing. At one point, the defendants' family sobbed. Lowery's wife also appeared distraught as the arraignment unfolded.

Defense attorneys asked the judge to prevent the public from seeing their clients in court. Prosecutor George Modlin fought the request, arguing the brothers made statements that placed them at the scene.

"This was a brutal, heinous, just cold-hearted case, and as the evidence comes out, I’d say the murder charges and the torture charges are very much warranted in this case," said Modlin.

In a post on an online fundraising site, Lowery's family described him as someone who didn't have much but still managed to help others.

"If someone was in need he would do what he could to help, no matter how hard the task. If he came across good fortune, he shared it," they wrote.

Anyone with information about the incident can call the homicide detail at 858-974-2321 or after hours at 858-565-5200.