Dean told a news conference in the parking lot of the Borderline Bar & Grill, "It's a horrific scene in there. There's blood everywhere."
The shooter has been identified as 28-year-old Ian David Long, who was also found dead.
The massacre was the deadliest mass shooting in the United States since 17 classmates and teachers were gunned down at a Parkland, Florida school nine months ago. It also came less than two weeks after a gunman killed 11 people at a synagogue in Pittsburgh.
US President Donald Trump said on Thursday via Twitter that he had been "fully briefed on the terrible shooting". He praised law enforcement, saying "Great bravery shown by police" and said "God bless all of the victims and families of the victims."