Gray, 25, who died one week ago after his spine was nearly severed following his arrest, was to be buried tomorrow in his hometown of Baltimore, less than an hour from the US capital Washington.
A viewing of his body for friends and family was to be held at a local Baltimore funeral home beginning at 1:00 pm local time today.
One day earlier, protesters rampaged through downtown Baltimore, in the latest show of public rage after the death of an unarmed African American by US law enforcement.
"We've got to take this department apart and try to figure out what is wrong and what is right," Cummings told CBS television's "Face the Nation" program.
Also Read
"This is a significant moment," Cummings said. "If we don't correct this now, it will only get worse."
News reports said the demonstrations drew thousands of protesters, in the biggest show of outrage so far over Gray's death.
But the mood shifted dramatically when scores of protesters moved to the vicinity of the city's Camden Yards baseball stadium, scene of an evening Baltimore Orioles-Boston Red Sox game.
Gray's twin sister Fredericka appealed -- in vain -- for calm.
"My family wants to say: Please, please stop the violence. Freddie would not want this," she said amid the violence that led to numerous arrests.
She spoke alongside Mayor Stephanie Rawlings-Blake, who said she was "profoundly disappointed" by violence she blamed on "a small group of agitators."
Street signs were torn down, and one motorist got a rock through her car window, local news media reported.
Targets of the violence also included a convenience store, a Michael Kors fashion boutique, a financial services center and a cellphone shop in Baltimore's landmark Lexington Market.