"He was ready to fight," Zogby said.
The hostages, identified as Zoeb and Sarrida Nek, were shaken up but not hurt, he said.
Police and neighbors described Vargas as a quiet man who had only recently moved into the building.
Tenants painted a mixed portrait of the gunman.
"He was a good son," said Ester Lazcano, who lived on the same floor as Vargas and his mother. "He'd take her in the morning to run errands" and to doctor appointments.
Lazcano said she was in the shower when she heard the first shots, then there were at least a dozen more. "I felt the shots," she said.
Valdes said Vargas was also known as a difficult person who sometimes got into fights and yelled at his mother.
"He was a very abusive person," she said. "He didn't have any friends there."
Zogby called Vargas' background "unremarkable." Police had not responded to any prior calls at his home.
"Nobody seems to know why he acted the way he acted," Zogby said.
As police investigated the crime scene, relatives of the victims began arriving to pick up their loved ones' belongings. Residents came out of their complex and spoke among each other as the sky turned dark and threatened to rain. Some had large swaths of water in their apartment from the firefighters who responded to extinguish the blaze.
Vargas' apartment door and the ceiling outside it were charred.
Agustin Hernandez, Merly Niebles' brother-in-law, loaded several old pictures and other items from his relative's apartment in a grocery cart and into his car. One showed his teenage niece smiling in a red graduation gown. Another pictured his sister-in-law posing in a white dress and pearls.
A binder also from the apartment had pop artist Justin Beiber's name on the spine, presumably belonging to the teenage girl, who family members identified as Priscilla Perez.
Marcela Chavarri, director of the American Christian School, said the Perez was about to enter her senior year.
"She was a lovely girl," Chavarri said through tears. "She was always happy and helping her classmates."
In Hialeah — a suburb of about 230,000 residents, about three-quarters of whom are Cuban or Cuban-American — the block around the apartment building was closed off with crime scene tape. At about midday, officers removed a body from the building and carried it away in a van.
Detectives, meanwhile, tried to piece together every shot and every minute of what had happened.
"It could have been a much, much more dangerous situation," Zogby said.