(CNN) -- The 43-year-old gunman who set fire to his apartment in suburban Miami and then fatally shot six people in and around the building had given authorities no reason to believe anything was amiss until Friday night, police said Saturday.
"Nobody seems to know why he acted the way he acted," Hialeah Police Department Spokesman Carl Zogby told reporters about Pedro Vargas.
Police were looking into the possibility that Vargas -- described by some as a "quiet" man -- may have had a dispute with the building managers, who were among his victims, Zogby said.
But that had not been established. An initial check indicated that Vargas had no criminal history and no history of irrational behavior. "We don't have an explanation," Zogby said.
The incident began around 6:30 p.m. Friday, when callers to 911 reported that smoke was emanating from Apartment 408, which Vargas had recently rented and where he was living with his mother, Zogby said.
The Hialeah police spokesman said Vargas used a flammable liquid to spread the fire. As the two building managers ran toward the smoking apartment, Vargas exited the apartment and shot them both several times, killing them, Zogby said.
He returned to his apartment balcony, shooting 10 to 20 times into the street. A 33-year-old man who was returning home from work was struck in a parking lot across the street, Zogby said. He identified the slain victim as a father.
"The subject then ran out of his apartment, ran down to the third floor, kicked the door open of an apartment and shot a 69-year-old man, his 51-year-old wife and their 17-year-old daughter inside the apartment," Zogby said. All three died.
"Vargas then ran out of the apartment, ran throughout the building firing shots at random in a very irrational fashion," Zogby said.
By now, firefighters had arrived at the approximately 90-unit building but were unable to enter the burning apartment because of the gunfire, Zogby said.
Hialeah police -- aided by officers from nearby police agencies -- responded in droves, with more than 100 at the building, he said. "Our main objective at that point was to neutralize an active shooter," he said. "He fired at us and we fired at him."
After eluding police for about three hours, Vargas entered another apartment, where he holed himself up with two hostages and began negotiating with police, Zogby said.
But when the negotiations failed after about three hours, "the SWAT unit made the decision to enter and neutralize -- and shoot -- Mr. Vargas," Zogby said. "When they entered the apartment, he resisted, he fired at them. There was an exchange of gunfire, he was ready to fight."
The officers returned the fire, and Vargas died on the scene early Saturday, Zogby said.
The hostages were "shaken up," but not hurt.
Police found a 9 mm pistol and "plenty of ammo," Zogby said.
The fire never spread beyond the Vargas apartment.