An Arizona author, vegan chef and motivational speaker is charged with three counts of first-degree murder after he gunned down his ex-wife and their two children on Christmas Day, police officials said.

Anthony Milan Ross, 45, of Phoenix, is also charged with two counts of aggravated assault, 19 counts of aggravated assault against an officer and a single count of discharging a firearm in city limits, ABC 15 in Phoenix reported. He is being held without bond in the Maricopa County Jail.

Ross, who earned a following after losing a significant amount of weight and attributing the loss to his vegan diet, is accused of killing Iris Ross, 38, and their two children, Nigel, 11, and Anora, who was 10 months old. He is also accused of shooting at police officers during a six-hour-long standoff following the slayings.

Ross, who goes by his middle name, on Monday posted a video showing him and his son singing together for the holiday. The video was shot the night before.

"Hey guys, it's Milan, and I'm hanging out with Nigel on Christmas Eve," Ross said in the video. "We're about to embarrass ourselves. We're going to sing."

A giggling Nigel joined his father in singing “Santa Claus is Coming to Town,” laughing throughout the song and looking up at his father with adoration in his eyes.

Just over two hours after Ross posted the video, Nigel, his baby sister and their mother were dead, according to police.

The Arizona Republic reported that court documents state that witnesses saw Milan Ross shoot Iris Ross outside his apartment complex as she tried to run from him. Milan Ross then went into his apartment and the witnesses heard multiple gunshots, the Republic reported.

That is when police believe he killed Nigel and Anora. Iris Ross had gone to her ex-husband's apartment to pick the children up for a Christmas party at their own home about a mile away, the Republic reported.

The witnesses told investigators that Ross came back outside and began rifling through Iris Ross’ pockets. When the witnesses asked if she was OK, Milan Ross said “no” and pointed his gun at them, the court documents say.

When officers arrived at the home at 3:45 p.m., they found Iris Ross’ body on the patio. Ross barricaded himself in the apartment for several hours before he began firing at officers outside.

Ross initially told police negotiators that he had already killed the children, the Republic reported. They continued negotiating with him, however, in the hopes that one or both of the children had not yet been slain.

The suspect later changed his story, telling the officers that the children were still alive and that he would kill them, police officials said.

Just after 10 p.m., police officers sent a robot into the apartment, where a camera showed the baby dead in one of the rooms, the Republic reported. That was when a police team went in, at which time Ross opened fire on the officers.

He was taken into custody uninjured, but a 46-year-old officer was injured by shrapnel and was hospitalized in stable condition.

Nigel’s body was found in another room of the apartment after his father was arrested, the newspaper said.

Iris Ross' sister, Mary Wogas, told ABC 15 that she received a text from Milan Ross on Christmas afternoon, in which he admitted what he had done.

"I received a text yesterday about 2:52 p.m.," Wogas told the news station. "Two messages, (the) first one saying, 'I just killed Iris and the kids.' The second message was, 'with a gun.'"

Wogas said she had suspected that Milan Ross was unstable, but had no idea how her sister’s life, and those of the children, would end.

“I knew he had a very, very dark side,” Wogas said. “I didn’t know it was this dark.”

Wogas said that Milan Ross’ behavior changed in June, after Iris Ross filed for divorce.

"The man was off his rocker. One minute, he was yelling at her and calling her a whore and the next minute wanting to tell her he loved her and wanted them to be a family unit," Wogas told the news station. "He would also claim that he was working with a voodoo priest in New Orleans to put a hex on her. Just very, very dark, very dark intentions."

Milan Ross' Facebook page is full of inspirational posts about weight loss and a vegan diet. He has a website devoted to veganism called Full Flavor Vegan and co-authored two books, The Change and The Change Cookbook.

Iris Ross worked as an administrator at a local outpatient surgery center, where officials released a statement mourning the “beloved” executive and her children.

"Iris was a fabulous person with a huge heart, and we are honored to have had her as part of our family," the statement from St. Joseph's Outpatient Surgery Center read.

Iris Ross’ roommate, Lushanya Echeverria, told the Republic that Iris saw the beauty in everyone.

“She saw people on the inside,” Echeverria said. “Every person who encountered her, they felt touched by her. They felt seen.”

Echeverria, who shared a home with Iris Ross for about a year, described the children as “beautiful, bring lights.”

She said that Milan Ross had harassed his ex-wife in recent weeks, and had shown up to pick up the children with a handgun that he said he carried for protection, the Republic reported. Despite the problems, Iris Ross tried to see the positives in the situation.

“She believed everybody was honest and good, even her ex-husband,” Echeverria told the newspaper. “That ultimately, he would do right for the children and he would have a healthy parenting relationship with her so that the kids could grow up healthy.”

Milan Ross told KPNX-TV just last month that his son was his inspiration for losing more than 300 pounds. He said he weighed in at 518 pounds during a family trip to Universal Studios, when he was told he was too big to fit on a Harry Potter ride that Nigel wanted to go on.

"He started crying," Ross told the television station in the interview. "It was at that moment I learned that the decisions I was making didn't just affect me."

Losing the weight strengthened his bond with his son, Ross said. It also led to appearances in magazines and on television shows and to the writing of his two books.

Ross' fans weighed in on the slayings in comments on the Christmas Eve video he posted hours before he is accused of killing his family.

“This just shows you that you never know what state of mind someone is in,” one man wrote.

Others wondered if Ross’ vegan diet played a role in his actions.

“I support vegan ideals 100 percent, but unfortunately most of us are genetically wired to need omega 3 and some animal fat for a healthy brain function,” another commenter wrote.

“This has nothing to do with being vegan,” one woman responded. “Most serial killers and murderers…are meat eaters. This is about mental health, not diet!”

Others expressed anger that Ross allegedly killed his children just hours after posting such a happy video.

“Omg (Oh my God), I’m in shock, like wth (what the hell)?” one woman wrote. “How could someone do this to their family? I don’t even know what to say! I thought you were an inspiration, but I clearly was wrong! Rest in peace, little ones.”

Another woman wrote that killing an infant was unfathomable.

“Killing his wife is one thing, but to have a gun and go there to kill her, then go into a room and pull the same gun on a 10-month-old baby is another,” the woman wrote. “His poor son. He was old enough to understand what his daddy was doing. God rest their souls.”