But the Sydney Roosters prop, who has pleaded guilty to contrary conduct, claims he did not intend to racially vilify Indigenous five-eighth Mam by calling him a "monkey" mid-game on Sunday (AEDT).
Brisbane's Mam lodged a formal complaint to referee Adam Gee after Leniu targeted him during a scuffle late in the Roosters' 20-10 win at Allegiant Stadium.
Leniu has since been referred directly to the NRL judiciary and is expected to receive a long suspension at next week's hearing.
The NRL has so far been unable to source audio of the slur, but Broncos players are expected to testify when Leniu fronts the panel.
The Roosters recruit initially downplayed the incident when quizzed by broadcast media after the match, saying the tensions between the sides had only been "fun and games".
Those comments drew the ire of Indigenous players such as Kotoni Staggs and Latrell Mitchell.
Leniu said he was remorseful in the Roosters' first formal statement on the incident, released early on Thursday afternoon.
"I want to apologise to Ezra and his family for using the word I did and I am sincerely sorry to cause him such distress,'' Leniu said.
"I've put my hand up and want to take ownership of this. I said the word but I didn't mean it in a racist way. Anyone who knows me knows that's not who I am."
The Broncos, who have supported Mam throughout the ordeal, are expected to release a statement of their own later on Thursday.
The comment left Mam distraught and sparked a further confrontation between players from the Roosters and Broncos at the teams' Las Vegas hotel.
In the days since, current and former players have weighed in as the incident threatens to overshadow the league's highly publicised foray into the American market.
Roosters chief executive Joe Kelly apologised to Mam as part of the club's statement.
"Spencer has been completely open and honest throughout this process, and we will continue to support him," he said.
"We extend our deepest apologies to Ezra, his family and the wider Aboriginal and Torres Strait Islander community and offer our support to all parties during this time.
"We respect the judiciary process. Accordingly, the club will not be making any further comment prior to the hearing."Â
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