ISIS execution videos strikingly similar

The victim's last words are seemingly scripted by his captors.

The masked man speaks. He lifts the blade. The video fades to black.

ISIS has released video in which it executes David Haines, the third Western captive beheaded by the militant Sunni group.

It's titled a "message to the allies of America," similar to what ISIS called the first two videos.

The semantics of all three videos have slight differences, but the choreography is strikingly alike.

The executioner in the video posted Saturday appears to be the same person, speaking in what sounds like the same British accent as the man who purportedly killed American journalists James Foley and Steven Sotloff.

"It sounds, and he looks very much, like the same executioner that we've seen in the two previous killings," CNN's Nic Robertson said. "That accent there is unmistakably British."

The executioner is dressed identically in all three videos; black garb covering everything almost but his eyes, a leather strap with a pistol. In the first two videos, you could see his combat boots, but not in the most recent tape.

He appears to be of similar build and height. He waves a knife in his left hand, as did the militant in the previous two videos.

The first portion of the video starts off as the other ones did -- with a clip of a Western leader stating his support in the fight against ISIS. This time, it is British Prime Minister David Cameron explaining his backing of the coalition supporting the Peshmerga in northern Iraq.

Before the masked killer takes the life of his captive, he addresses the leader of his victim's country, referring to recent airstrikes conducted by the United States in Iraq.

Prior to Foley's killing, the executioner refers to U.S. strikes that helped break the siege of the Sinjar Mountains in an attempt to rescue the Yazidis trapped there. Prior to Sotloff's killing, the executioner refers to the strikes that helped break the siege of Amerli, home to many of Iraq's Shiite Turkmen.

This time?

"Your evil alliance with America which continues to strike the Muslims of Iraq and most recently bombed the Haditha dam, will only accelerate your destruction and claim the role of the obedient lap dog," he said, directing his remarks at Britain. "Cameron will only drag you and your people into another bloody and unwinnable war."

At the end, the executioner threatens the life of another captive -- this time, ISIS claims it has Alan Henning, another abducted British aid worker.

"This is stereotypical for what we've seen," Robertson said. "Unfortunately, this has to be said, that the outlook at the moment for Mr. Henning is not a strong one."

Following Foley's death, the British ambassador to the United States said that experts in his country were close to identifying his killer. He has not yet been named.

"The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude," Cameron said late Saturday. "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

In the case of the first two videos, the U.S. intelligence community has been analyzing them to try to answer some key questions: When were they shot? Where were they shot? Is the killer the same person?

One forensics expert has raised the issue that there appear to be two militants in the Foley video. The second militant appears on the video after an obvious edit.

"There's definitely a change of actor," said Ross Patel, the forensics expert. "There are noticeable -- there are subtle -- but there are also noticeable changes in their build, their physical appearance."

In the Sotloff and Haines video, the picture fades to black immediately before and after the start of the beheading so, again, it's not immediately clear whether the man speaking is the same man who then killed the journalist.