While investigators pore over about 100 leads, the FBI, Florida Department of Law Enforcement and Orlando Police said at a news conference notifying next of kin is their top priority.
Describing the murdered and wounded patrons as "followers of Lot," a spokesman hailed Omar Mateen and his bloody "incursion."
City officials have released the names of 49 people killed at the gay nightclub in Orlando Sunday.
Speculation fills the internet as officials determine if this is a hate crime, domestic terrorism, or both.
Some politicians with antigay records appear genuinely moved.
As the head of The GLBT Center of Central Florida, Terry DeCarlo is a well known figure in Orlando's gay, lesbian, bisexual and transgender community, and his heartbreak is apparent.
A national group is challenging the new law signed by Louisiana's Gov. John Bel Edwards that classifies attacks on police officers as hate crimes.