U.S. authorities say Omar Mateen, the man who carried out the worst mass shooting in modern U.S. history, had touted support not just for the Islamic State but also other militant factions, which are enemies of the Sunni extremist group.

At one point, Mateen expressed solidarity with a suicide bomber from the Syrian branch of al-Qaida, which is known as the Nusra Front and which is the Islamic State group's top rival. And a few years back, he claimed connections to the Lebanese Shiite Hezbollah group as well.

Here's a look at the groups that Mateen is said to have espoused — all of them listed by the United States as terrorist organizations.

THE ISLAMIC STATE GROUP

The Islamic State group is a Sunni Muslim group that controls vast territory stretching across northern Syria, through much of northern and western Iraq, where it declared a "caliphate" in 2014. The group enforces its own radical interpretation of Islamic law, or Shariah, over territories it controls and terrorizes residents into submission. IS has claimed responsibility for many deadly attacks around the Middle East and Europe but most of its victims have been other Muslims whom it deemed as traitors or insufficiently pious.

The group has also beheaded a number of Western journalists and aid workers. Its deadliest attacks outside Syria and Iraq were in Paris in November, in which 130 people died, and in March in Brussels, an attack that left 32 people dead.

The Islamic State group's Al-Bayan Radio described the Orlando shooter as "one of the soldiers of the caliphate in America."

THE NUSRA FRONT

The Nusra Front is al-Qaida's branch in Syria and, like the Islamic State group, has among its fighters many foreign jihadis — including Europeans, Americans, Arabs and Asians. Still, when the Nusra Front split with IS more than two years ago, most of its foreign fighters joined its rival while most Syrians remained among its ranks.

Like the IS, the Nusra Front is a Sunni militant group that has frequently come under attack by U.S. forces. The Nusra Front and IS have claimed responsibility for deadly bombing attacks targeting Shiite-majority areas in Lebanon and have killed and wounded scores of people since 2013.

Mateen pledged loyalty to Nusra Front's suicide attacker Moner Mohammad Abusalha, an American who grew up a basketball fan in Vero Beach, Florida. Abusalha killed himself in May 2014 along with 16 people in a suicide bombing targeting Syrian government forces in the northwestern province of Idlib.

Al-Qaida, which was founded by Osama bin Laden of Saudi Arabia in the late 1990s, is the mother group of both IS and the Nusra Front. The groups merged briefly to form the Islamic State in Iraq and the Levant but split shortly afterward and eventually IS denounced al-Qaida.

LEBANESE HEZBOLLAH GROUP

The Shiite Muslim Hezbollah group has been one of the main forces backing Syrian President Bashar Assad's troops after Syria's conflict began in March 2011. Hezbollah has been fighting against the Nusra Front and the Islamic State group mostly in areas near the border with Lebanon as well as in northern Syria.

The group's Shiite ideology is different from the Salafi jihadi brand that Nusra Front and Islamic State group espouse. The IS and al-Qaida consider Shiite Muslims apostates whose blood may be shed.

The Iran-backed Hezbollah was formed in 1982, mainly to fight Israeli occupation of parts of Lebanon, and has fought several wars against the Jewish state. It has sent thousands of its fighters to Syria to defend Assad's government which has long provided logistical and other support to the group.