Foley's parents, flanked by one of his brothers, talked to reporters about their son's plight.
"Jim was innocent and they knew it," his mother, Diane, said. "They knew that Jim was just a symbol of our country."
His father broke down several times.
"We beg compassion and mercy" for the other American journalist shown in the video, said John Foley. Sotloff, a contributor to Time and Foreign Policy magazines, was kidnapped at the Syria-Turkey border in 2013.
"They never hurt anybody," John Foley said. "They were trying to help. There is no reason for their slaughter."
James Foley, 40, previously had been taken captive in Libya. He was detained there in April 2011 along with three other reporters, and released six weeks later.
Afterward, he said that what saddened him most was knowing that he was causing his family to worry.
His parents talked about asking him why he wanted to return to conflict zones.
"Why do firemen keep going back to blazing homes?" John Foley told reporters. "This was his passion. He was not crazy. He was motivated by what he thought was doing the right thing ... that gave him energy to continue despite the risk."
His mother said she remembered him telling her, "Mom, I found my passion. I found my vocation."
Source: Foley tortured, beaten
Disturbing details about Foley's final months began to emerge Wednesday.
A source who claims to have been held last year with Foley told CNN's Bharati Naik that he, Foley and another journalist were held from March to August 2013 in a prison in the Syrian city of Aleppo near Masha al-Adfaa hospital.
At the time, the source -- who spoke on condition of anonymity -- said they were being held by al-Nusra Front, a Syrian rebel group with ties to al Qaeda in Iraq.
At one point, according to the source, there were almost 100 people -- including other European journalists -- in the prison.
The source believes Foley and the other journalist, who was not Sotloff, were transferred to an ISIS training camp.
Foley and the other journalist, according to the source, were tortured in prison -- mostly beaten.
Foley and the other journalist, who the source declined to identify, said they gave him contact numbers and e-mail addresses to pass on messages to their family members.
The source told CNN he lost the contacts and did not get in touch with the families. He said he did, however, give the information about the journalists to Western government authorities in November 2013, including details about where Foley was being held.
French journalist Nicolas Henin told France Info radio he had been held with Foley in northern Syria prior to his release in April.
Henin, who has never before spoken about Foley because he didn't want to jeopardize his safety, said he was held for seven months with the American journalist.
Hostages were held in groups. At one point, he shared a cell with Foley.
Foley "was in a difficult state," Henin said. "He already suffered a lot during his first months (of captivity) and thankfully we shared a phase (in our detention) that was less difficult."
Foley, according to Henin, said he had been initially kidnapped by a group of jihadists who were fighting in Syria.
The Committee to Protect Journalists estimates there are about 20 journalists missing in Syria, many of them held by ISIS.