ISIS leader Abu Bakr al-Baghdad is dead

The families of U.S. citizens who were murdered by Islamic State of Iraq and Syria chief Abu Bakr al-Baghdadi on Sunday called his death a "victory." Mr. Trump said in his address Sunday morning that ISIS "murder of innocent Americans Jim Foley, Steven Sotloff, Peter Kassig, and Kayla Mueller were especially heinous."

President Trump announced al-Baghdadi's death at a press conference Sunday morning. During a special operation that resulted in U.S. forces pursuing al-Baghdadi and three of his children in a "dead-end tunnel," the ISIS leader detonated a suicide vest.

Al-Baghdadi has been accused of taking at least three U.S. civilians hostage and murdering them. According to the White House, the ISIS leader is responsible for more than 300 public beheadings and for killing thousands of captured prisoners of war.

Kayla Jean Mueller Mueller family photo

White House press secretary Stephanie Grisham said in a statement that Mr. Trump called family members of those killed by ISIS, but said they are keeping the details of the conversations private.

The operation that resulted in al-Baghdadi's death was named in Kayla Mueller's honor.

Mueller, who was kidnapped and taken hostage by ISIS in 2013 while helping Syrian civil war refugees, is credited for saving the lives of other female captives. She was killed in 2015.

Although the exact circumstances surrounding her death are unknown, Mueller's parents believe that al-Baghdadi "either killed her or he was complicit in her murder," according to AZ Central.

"I still want to know, 'Where is Kayla?' and what truly happened to her and what aren't we being told," said Marsha Mueller, Kayla's mother after al-Baghdadi's death. "Someone knows and I'm praying with all my heart that someone in this world will bring us those answers."

Peter Kassig, the American founder of the relief organization Special Emergency Response and Assistance (SERA) and an Iraq War veteran, in an undated photo. SERA

Mueller was captured around the same time as American aid worker Peter Kassig, who had started an organization called Special Emergency Response and Assistance (SERA) to help Syrians escaping the country's ongoing war.

He was captured by ISIS In 2013 and beheaded a year later, just a few weeks after journalist James Foley received the same fate.

Foley went missing in northwest Syria in 2012. He was beheaded in a video by ISIS members two years later.

James Foley is seen in Benghazi, Libya, in a still from a 2011 video. GlobalPost/AP

Journalist Steven Sotloff is seen at the end of the same video when an ISIS member says Sotloff will die if the U.S. does not cease attacks in northern Iraq. Sotloff had been missing since July 2013 and was beheaded a few weeks later.

Sotloff's parents, Art and Shirley Sotloff, told CBS News they are thankful to Trump, the U.S. Special Forces and all those involved for killing al-Baghdadi.

"While this victory will not bring our beloved Steven back to us, it is a significant step in the campaign against ISIS," they said. "It is our hope that our son's surviving captors, nicknamed "the Beatles," will be brought to justice, that all remaining hostages are returned to safety, and that the United States will take every measure to eliminate the resurgence of ISIS and terror in all forms."

American journalist Steven Sotloff, center with black helmet, talks to Libyan rebels on the Al Dafniya front line 25 kilometers west of Misrata, June 2, 2011, in Misrata, Libya, in this handout image made available by the photographer Etienne de Malglaive via Getty Images

Diane Foley, James Foley's mother, provided CBS a similar sentiment.

"I am grateful to our president and brave troops for finding ISIS leader al-Baghdadi. I hope this will hinder the resurgence of terror groups and pray that captured ISIS fighters will be brought to trial and held accountable," she said.

She added that she remains concerned about other Americans who are believed to be held hostage by the terrorist organization, including journalist Austin Tice, who was abducted in Syria in 2012 while documenting the country's civil war.