The only child of the late singer Whitney Houston and Bobby Brown remains in a coma.
Citing unnamed law enforcement sources, TMZ reports the investigation centers on possible foul play.
The target of the investigation is said to be her boyfriend, Nick Gordon.
Bobbi Kristina Brown was found in a bathtub at her Georgia home last week.
An attorney said Wednesday that his client, Maxwell Lomas, 24, of Duluth, Georgia, found Bobbi Kristina Brown face-down in a bathtub last week and called 911 at a townhome in suburban Atlanta.
Lomas and the 25-year-old Gordon were both listed in the police report as being at the home when investigators arrived.
"He immediately pulled her from the water and called 911 as well as alerting other people in the house," attorney Philip Holloway said of Lomas. "He did whatever he could do to assist in resuscitative measures until first responders arrived."
Previously, police in Roswell, Georgia, where the townhome is located, had said that Gordon and Lomas found the woman in the bathtub and that Lomas called 911 while Gordon performed CPR. Gordon has not spoken publicly since then.
The incident happened Jan. 31, less than a month before the third anniversary of when Houston was found unresponsive in a bathtub and later died. Authorities found a dozen prescription drug bottles in her Beverly Hills Hotel suite and listed heart disease and cocaine use as contributors, but concluded that she accidentally drowned on Feb. 11, 2012.
On Jan. 15, 2015, Lomas was arrested and charged with weapons offenses, possession of marijuana with intent to distribute, and possession of a prescription anxiety medication. Lomas pleaded not guilty to the charges, according to Holloway, who said the arrest is unrelated to Brown being found in the bathtub.
"He remains in the Metropolitan Atlanta area and has been questioned by police as have many other people," Holloway said Wednesday. The attorney described Lomas as a close friend and frequent house guest of Brown's.
Bobbi Kristina Brown has commented often about her marriage to Gordon, the orphaned young man she grew up with after Houston brought him into the family. But her father's lawyer says they never wed.
"To correct earlier reports, Bobbi Kristina is not and has never been married to Nick Gordon," reads a statement released Tuesday by R&B singer Bobby Brown's attorney, Christopher Brown.
Legal authority over Bobbi Kristina's health care and money could become a critical issue for the fractious Houston-Brown family, which has said that Bobbi Kristina "is fighting for her life" in a hospital.
Houston never formally adopted Gordon, but raised the two teenagers together after divorcing Brown.
Bobbi Kristina became the sole inheritor of her mother's estate when she died in 2012, and shortly thereafter, she and Gordon went public with their romantic relationship.
Then, on Jan. 9, 2014, Bobbi Kristina posted a photo on Twitter of the couple's hands wearing wedding rings, with the caption "#HappilyMarried. So #InLove. If you didn't get it the first time that is."
And as recently as Dec. 25, she was still telling the world that Gordon is her husband:
"MerryChristmas!! :)(:Snuggling in a beautiful hotelroom with MYbeautiful husband nickdgordon :)(:Life couldn't be sweeter at the moment!"
Relations between Gordon and some other family members soured last year: He remains subject to a protective order barring him from being within 200 feet of Bobbi Kristina's aunt, Patricia Houston.