US aircraft carrier strike group arrives in Middle East

The Syrian Civil Defence, also known as the White Helmets, said it had concluded its search for detainees within Sednaya Prison on Monday after failing to uncover any "unopened or hidden areas" within the facility.

The prison previously held thousands of people detained by the former regime of ousted President Bashar al-Assad. Families of the missing and survivors believed that some detainees might have been unable to leave over the past two days, potentially due to being held in tightly sealed and secured areas, those close to the situation told the White Helmets, prompting the search.

"Specialized teams conducted a thorough search of all sections, facilities, basements, courtyards, and surrounding areas of the prison," the White Helmets said in a statement. "These operations were carried out with the assistance of individuals familiar with the prison and its layout. However, no evidence of undiscovered secret cells or basements was found."

The statement also called on international organizations and local authorities to support the efforts of the Syrian Civil Defence in uncovering the fate of the detainees and returning them to their loved ones.

"We share the profound disappointment of the families of the thousands who remain missing and whose fates remain unknown," the statement continued. "We stand in solidarity with the victims’ families, fully understanding their anguish and their longing for answers about their loved ones."

-ABC News' William Gretsky

Secretary of State Antony Blinken said senior officials from his department are "fanning out through the region as we speak" to work with their counterparts on how the U.S. can "help support the Syrian people as they decide their own path for the future."

"We have a strong interest in preventing the reemergence of ISIS, given the death and destruction that it has wrought for so long,” he said during remarks at an unrelated event on Monday.

Blinken noted that ISIS would seek to exploit the moment, and that U.S. strikes on ISIS sites over the weekend demonstrate that the U.S. is "determined not to let that happen."

"We have a clear interest in doing what we can to avoid the fragmentation of Syria, mass migrations from Syria and, of course, the export of terrorism and extremism," he said. "The region and the world have a responsibility to support the Syrian people as they begin to rebuild their country and charge a new direction."

Blinken also said that with every party they engage with, he and other U.S. officials will continue to seek information on American freelance journalist and Marine Corps veteran Austin Tice, who went missing while covering the civil war in August 2012, "so that we can find him and bring him home to his family and loved ones."

-ABC News' Shannon K. Kingston

The special presidential envoy for hostage affairs is in Lebanon as the Biden administration tries to capitalize on the fall of the Assad regime to uncover information on the whereabouts of missing American freelance journalist and Marine Corps veteran Austin Tice, according to a U.S. official.

The envoy, Roger Carstens, was in Doha last week but traveled to Beirut when the Assad regime fell, the official said. The Biden administration is working through multiple partners in the Middle East -- most notably Lebanon and Turkey -- to track people coming out of Syrian jails.

However, U.S. officials say they still have very little intelligence on Tice's whereabouts and can't say with certainty that he is even in Syria.

Tice went missing while covering the civil war in August 2012.

-ABC News' Shannon K. Kingston

The Syria Civil Defence, aka the White Helmets, announced Monday it is offering a $3,000 reward for any direct information that will lead them to Assad's "secret" Syrian prisons.

The organization addressed former security officers and those working in the security branches for help in accessing the prisons, adding they will maintain the confidentiality of sources.