Ilan Troen was on the phone with his daughter in Israel as she was shot and killed by Hamas gunmen while shielding her son from their bullets, he said.

Troen, a professor emeritus from Brandeis University in Massachusetts, said his daughter and son-in-law, Deborah and Shlomi Matias, were killed by Hamas militants over the weekend. Troen's grandson, 16-year-old Rotem Matias, was shot but will survive, Troen told CNN's Poppy Harlow on Monday.

The killings came after the Gaza-based militant group launched devastating attacks on Israel early Saturday.

Deborah Matias is one of at least nine US citizens confirmed dead in the ongoing conflict.

"At this time, we can confirm the death of nine U.S. citizens. We extend our deepest condolences to the victims and to the families of all those affected, and wish those injured a speedy recovery," a National Security Council spokesperson said in a statement Monday.

State Department spokesperson Matt Miller said the department is offering "consular assistance" to the victims' families.

As desperate families continue to wait for information about loved ones in the region who are missing, Troen said he has "too much information" about what happened when the gunmen burst into his daughter's home.

"We were on the phone with Deborah as she was killed," Troen said.

"We were on the phone the entire day with our grandson, Rotem, as he lay first under her body, and then found a place to escape under a blanket in a laundry."

Rotem was shot in the stomach, Troen said, but will recover.

"The brunt of the shot was borne by his mother," he said.

"The terrorists who came, they had explosives and blew up the front door to their house and then blew out the front door to their so-called safe room."

Rotem hid for more than 12 hours after he was shot, texting on his phone to communicate with people who were coaching him on how to breathe and how to manage "the blood that was coming out of his abdomen," Troen said, adding Rotem's phone was down to a 4% charge when he was rescued.

Deborah Matias attended the Rimon School of Music in the Tel Aviv area, where she met her husband, Troen told CNN.

"Deborah was a child of light and life," Troen said. "She, rather than becoming a scientist or a physician, she said to me one day, 'Dad, I have to do music, because it's in my soul.'"

Troen spoke to CNN from Be'er Sheva, Israel, where he said jet planes flew over his house into Gaza. "This is not a normal war," he said. "It isn't like there's a front and rear."

Troen said the last he heard, Rotem was with family in the hospital.

"He's 16, tough, resilient -- he survived this. He'll survive more, but the trauma of this is going to last his lifetime," he said.

Missing Americans

Jacob Ben Senior said his daughter Danielle was attending the Nova music festival near the Gaza-Israel border and has not been heard from since Friday. Ben Senior said he has been calling her phone since Saturday morning but has not been able to reach her.

Born in Los Angeles, Danielle Ben Senior is a 34-year Israeli-American citizen who has lived most of her life in Israel, according to her father. Danielle was working at the Nova festival with a group of event organizers, her father said.

"We are in close contact with the government of Israel as they continue to conduct security operations to locate missing US citizens," Miller, the State Department spokesperson, said.

The Biden administration is "laser-focused" on confirming whether any Americans have been taken hostage by Hamas, deputy national security adviser Jon Finer said during an appearance on CBS News earlier Monday. The US is prepared to offer "expertise on how to address these hostage situations," he said, with more information expected in the next couple of days.

Israel's Minister of Defense on Monday ordered the "complete siege" of Gaza, cutting off electricity, food, fuel and water to the enclave. This comes as Israel has pounded Gaza with airstrikes and formally declared war on Hamas on Sunday.

More than 550 Palestinians have died, according to Gaza's health ministry, and medical care has been complicated by Israel cutting power to the territory.

It's unclear whether any US citizens are among those in killed or injured in Gaza.