The attack early Sunday on an Orlando nightclub stands as one of the deadliest terrorist assaults on U.S. soil in recent years, with deaths and casualties topping those of shootings in San Bernardino, Calif., Chattanooga, Tenn., and Fort Hood, Texas, among others.

Orlando police said 51 people were dead, including the shooter, Omar S. Mateen, following the attack, with 53 others injured. The shooter had called 911 before the attack, pledged allegiance to Islamic State and then prayed in a foreign language on the phone, a person familiar with the investigation said. (See an interactive map and timeline showing how the Orlando attacks unfolded.)

The massacre followed a string of mass shootings and similar events in recent years, a number of which were found to be tied to Islamic extremism.

The couple that opened fire on an office holiday party last December in San Bernardino, Syed Rizwan Farook and Tashfeen Malik, were found to have ties to Islamic State and were radicalized as early as 2013, according to Federal Bureau of Investigation Director James Comey in Senate testimony after the shooting. The assault killed 14 people and injured 22 others.

In July 2015, Mohammod Youssuf Abdulazeez, a 24-year-old U.S. citizen, opened fire on two military facilities in Chattanooga, Tenn., killing five. Mr. Comey later called the shooting “inspired by a foreign terror organization,” though declined to say which one, saying “there are a lot of competing poisons out there.”