The absence of a threat comes as ISIS has now beheaded a total of five Westerners, including two other Americans, journalists James Foley and Steven Sotloff.
The video raises questions of who is still being held hostage by the Islamic extremists and what the terror network might do next.
Though ISIS, whose members refer to it as the Islamic State, didn't utter a new ultimatum, the radicals aren't expected to end their killing of hostages. In fact, they may be trying to step up schemes in the Mideast to kidnap more Westerners and parlay their captivity for ransom, experts say.
It's hard to know the exact number of captives because governments, employers and families tend to keep kidnappings quiet for fear of putting the victims in greater danger while negotiators work to secure their release.
Many missing journalists
After Foley's killing, a U.S. official told CNN
that ISIS was believed to be holding a number of Americans. The official, who spoke on condition of anonymity, declined to identify them or say exactly how many Americans were being held.
The Committee to Protect Journalists estimates that about 20 journalists are missing in Syria -- most of them local, some from outside Syria. It says many of them are believed to be held by ISIS.
Among them is Austin Tice, an American freelance journalist who was contributing articles to The Washington Post. He disappeared in Syria in August 2012. There has been no word of him since he disappeared.
As for the two known captives -- the British journalist and the American aid worker -- their fates are uncertain, even as more Muslim leaders join Western leaders and others in their condemnation of ISIS' gory slaughter of hostages.
'Going to hell'
Kassig's case was also different because he converted to Islam while in captivity and went by the first name of Abdul-Rahman.
This week, prominent Syrian Sunni cleric Shaykh Muhammad al-Yaqoubi declared that ISIS chief Abu Bakr al-Baghdadi "is going to hell." Al-Yaqoubi had been the imam in the Grand Umayyad Mosque in Damascus, but he had to flee Syria -- where ISIS is headquartered -- because the cleric called on President Bashar al-Assad to step down after the 2011 revolution began.
"I think we have to speak loud and very clear that Muslims and Islam have nothing to do with this," al-Yaqoubi told CNN. "There's no end unless we stop people from joining ISIS."
In fact, it's likely that ISIS will plot to take more hostages in the future, as it is a way for the group to show it is still fighting, analysts say.
If anything, the reach of ISIS may be spreading, with jihadist groups in Egypt and Libya pledging their support for the extremists.
They and ISIS may be targeting Western
travelers and tourists to those countries and the Mideast as the next potential hostages, analysts say.
"The swearing of allegiance to ISIS of the Egyptian jihadist group Ansar Beit al Maqdis last week may give the wider ISIS network new opportunities to take Westerners hostage because of tourist travel to Egypt," CNN terror analyst Paul Cruickshank said.
"There is also concern the group could try to snatch Westerners in Turkey and Lebanon and bring them back across the border to Syria," he said.
Cruickshank pointed out that an Algerian ISIS affiliate, Jund al Khilafa, kidnapped and beheaded French hiker Herve Gourdel,
55, in Algeria in September.
Militant groups elsewhere also purport to be acting in support of ISIS.
The Islamist militant group Abu Sayyaf in Philippines
kidnapped two Germans and threatened to kill them if the militants didn't get a $5.6 million ransom and if the West didn't back off of ISIS.
The two Germans, Stefan Okonek, 71, and his partner Henrike Dielen,
55, long-time residents of Palawan, were released in October after being abducted in May, CNN affiliate ABS-CBN reported.
U.S. reviews its response
The Obama administration is now reviewing the U.S. response to the hostage-taking of Americans by overseas terrorists.
A top Pentagon official wrote in a letter to Rep. Duncan Hunter, R-California, that President Barack Obama had ordered a "comprehensive review of the U.S. Government policy on overseas terrorist-related hostage cases, with specific emphasis on examining family engagement, intelligence collection and diplomatic engagement policies."
The review comes
as a result of "the increased frequency of hostage-taking of Americans overseas" and would seek to add "innovative and non-traditional solutions" to hostage-recovery efforts, Under Secretary of Defense Christine Wormuth wrote to Hunter.
Hunter has advocated for a tougher U.S. response when Americans are taken captive overseas.
Diane Foley, mother of the murdered American journalist, excoriated the U.S. handling of her son's abduction and stated: "I think our efforts to get Jim freed were an annoyance" to U.S. officials. Her son was killed in August.
After Foley's death, the U.S. government announced it tried to rescue him and other hostages in a military operation over the summer, inside Syria, but the Pentagon said the hostages had been moved.
$20 million in ransom collections
ISIS has found hostage-taking to be lucrative as other governments, particularly in Europe, have successfully won the release of captives by paying ransoms to ISIS.
Obama and other U.S. officials denounce the practice, saying the money has fueled the group's rise.
"Although ISIS is perhaps most notorious for executing the individuals it kidnaps, in particularly gruesome fashion by beheading them and then sending the video around on social media, the group also relies on kidnapping for ransom as a form of revenue generation," said terrorism expert Colin Clarke of the RAND Corporation.
In 2013, ISIS collected about $20 million from ransoms generated through hostage-taking, Clarke said.
"Now that ISIS only has two known hostages remaining in captivity, the group could seek to reach out to other rebel groups operating in Iraq and Syria to acquire more hostages," he said.
Another option would be for ISIS to begin working closely with any criminal gangs or kidnapping rings in the Mideast that are "more interested in profit than ideology," Clarke said.