U.S. Attorney General Loretta Lynch was due to meet on Tuesday with victims of the Orlando nightclub massacre, a day after the Federal Bureau of Investigation said the gunman had described himself as an "Islamic soldier" during the rampage.

Omar Mateen killed 49 people and wounded 53 others when he opened fire inside Pulse, a gay dance club, early on June 12 in the deadliest mass shooting in modern U.S. history. He was later killed by police after a three-hour standoff.

The 29-year-old U.S. citizen of Afghan descent pledged allegiance to the leader of the Islamic State militant group in a 911 call from the nightclub, but authorities said he appears to have been "self-radicalized" and to have acted alone.

Lynch was to visit Orlando on Tuesday to meet with survivors of the attack, victims' loved ones, first responders and others affected by the killings, the Department of Justice said.

She will also be briefed on the investigation, the department said, and address the media alongside U.S. Attorney A. Lee Bentley III of the Middle District of Florida.

In a transcript of the gunman's calls released by the FBI on Monday, Mateen told police negotiators to tell the U.S. government to stop bombing Syria and Iraq, and he threatened to strap bomb vests on hostages, though no explosives were found at the scene.

FBI Assistant Special Agent in Charge Ron Hopper told a news conference on Monday the shooter had made his "murderous statements" in a "chilling, calm and deliberate manner."