Brown made the claim on Facebook after it was revealed that Gordon would be taking part in an intervention that will be aired during an episode of The Dr. Phil Show on March 11. A source told PEOPLE that a "distraught" Brown had already checked into rehab following the appearance.
Brown's post, which was directed to Dr. Phil, took issue with the fact that Gordon was appearing on the show.
"Nick Gordon is under investigation for the attempted murder of my niece Bobbi Kristina Brown," she wrote. "We have strong evidence of foul play. Until this investigation is completed by law enforcement, I would ask that you or anyone else not provide this individual a platform to spin this situation to his benefit.
"If Nick Gordon does not have the courage to speak with my brother Bobby Brown and/or law enforcement about what happened the day my niece's body was found in a bathtub, he does not deserve to have a platform to speak to anyone of your caliber until this investigation is concluded," Brown continued.
Police had previously confirmed to PEOPLE that they have launched an investigation into the Jan. 31 near drowning of Bobbi Kristina, but they did not confirm if Gordon was directly under investigation.
Roswell Police Chief Rusty Grant said that the police are "going to follow the evidence and see where it leads us."
Gordon's feud with Bobbi Kristina's family – and specifically, Bobby Brown – intensified in late February when he took to Twitter to demand that Bobby allow him to see Bobbi Kristina in the hospital.
Bobby's lawyers fired back at Gordon, saying that he had failed to meet the terms of a possible visit, which reportedly included explaining the circumstances surrounding Bobbi Kristina's near drowning. Gordon's attorneys claimed that Bobby couldn't guarantee access to Bobbi Kristina, so they had advised Gordon to not meet those conditions.
"Per his own words, Mr. Brown has only 'offered an opportunity to potentially meet with Bobbi Kristina' if Nick were to meet certain conditions, conditions that we, his attorneys, have advised him not to meet, particularly in light of Mr. Brown's inability to guarantee access."
A family source previously told PEOPLE that "foul play is being investigated" including "drug use and possibly cover-up to remove evidence from the crime scene."
TMZ reported on Feb. 6 that Max Lomas, a friend who was with Gordon when they discovered Bobbi Kristina unresponsive in a bathtub, had seen Gordon cleaning blood stains.
Gordon and Bobbi Kristina first went public with their romance in 2012, shortly after Bobbi Kristina's mother, Whitney Houston, passed away. The pair dated on and off over the years and Brown referred to Gordon as her "husband" on social media. However, Bobby's lawyers say that the two were never legally married.
A source close to the family described Bobbi Kristina and Gordon's relationship as "toxic" and "codependent."
However, the source adds that Bobbi Kristina felt Gordon was the only person she could trust, "so even though they argued, she always went back to him."
Gordon's lawyer was unavailable for comment.
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