Branch's request was made during the latest in a handful of investigative hearings by the City Council's Public Safety Committee examining the case of Tyrone West, who died in police custody in July 2013.
Branch later said that he felt the committee had reached the limits of its power to probe into the events surrounding the death of West, along with those of Maurice Johnson, and Anthony Anderson.
"Police powers fall under the state of Maryland, there's nothing we can do," Branch said after the meeting. "The best remedy for me was to ask the state's attorney if she would reopen the [West] case, not only that case, but the other two cases, in light of the circumstances, especially in the Freddie Gray situation."
Branch formally invited Mosby's office to appear before the committee, but no representative was present at the meeting, leading some to wonder whether Branch's statements were merely a symbolic gesture.
"I'm glad that he's willing to offer to ask her, but I want to see it in black and white," Tawanda Jones, West's sister, said after the meeting. Jones has led a steady but so far fruitless campaign to have criminal charges brought against the officers involved in the incident leading to her brother's death.
"For two years, nobody did anything," she said. "We've been our own investigation. We've been just doing everything. I don't have faith in Baltimore City whatsoever."
Jones' faith and that of her family and their supporters was further tested on Tuesday when Branch announced that the committee would not be hearing public testimony. The announcement drew angry shouts from the audience and an extended scolding from West's aunt, Diane Butler, culminating in chants of "we can't stop, we won't stop, 'til killer cops are in cell blocks."
West supporters said they found it particularly distasteful that interim police commissioner Kevin Davis was given the opportunity to address the committee while they were not.
During the meeting, Davis offered his sympathies to the West family, saying that the results of an independent commission study into West's death, released in August of last year, had helped guide reforms in the department.
"We have become a better department because of this," Davis said.
Reached after the meeting had adjourned, Branch said he still had questions about the implementation of the study's suggestions, in light of recent events.
"For me to hear those recommendations from the previous cases, and then we still continue to deal with situations like with the Freddie Gray case, it sort of [raises] questions to me as to whether the recommendations are really being taken serious," he said.
As to whether there was anything else the council could do, Branch said, "I don't know, I don't have all the evidence in front of me. Only the state's attorney’s office can make that determination."
Johnson was shot to death in his mother's home in May of 2012. Anderson died after being thrown to the ground by police during an arrest in September of that year.
In both the Anderson and West cases, then-city state's attorney Gregg Bernstein conducted investigations that concluded the police's use of force was justified.
Mosby's office did not immediately return calls for comment for this article.