Brown, the only daughter of the late Whitney Houston
, remains in a medically induced coma in a neuro intensive care unit at Atlanta's Emory University Hospital.
Monday night, friend Daphne Barak said that Nick Gordon, Brown's boyfriend, told her that the bruises were a result of him performing CPR on her.
Gordon could not be reached for comment.
The investigation is focusing on everyone who was in the house the night before the incident and on people who had contact with Gordon, according to a source with knowledge of the probe.
Police are exploring all avenues to find out how and why Brown ended up face down in her bathtub with unexplained injuries to her body, the source said.
"Immediately, from the beginning, they asked him about injuries to the chest," Barak told HLN's Nancy Grace, adding that Brown is "very tiny."
Her father's attorney released a statement criticizing the media for "false reports."
"The desire to be 'first' has clouded the judgment of many reporters as they forgo accuracy. This is a criminal investigation and the integrity of that process requires silence," Christopher Brown said.
Police didn't call the case a criminal matter until Tuesday night.
"I can confirm that we have a criminal investigation ongoing," Roswell Police Chief Rusty Grant said in an email sent to CNN.
Friend: They were in love
Barak has been in close contact with the couple for more than a year, she said, adding that she believes Gordon and Brown were very much in love.
"These two young people -- he was everything for her, she was everything for him. ... They really were everything to each other," Barak said.
Last week, a source close to the Brown family told CNN that Brown has injuries that need to be explained, but there have been no details disclosed publicly about the nature of those alleged injuries.
"I'm praying for her," Barak said.
Houston died three years ago Wednesday. She was found dead in a bathtub at a hotel near Los Angeles on February 11, 2012, hours before she was to attend a pre-Grammy Awards party. A coroner ruled her death an accidental drowning, with heart disease and cocaine use listed as contributing factors.