Newly-released satellite images of the presumed site of an Israeli air raid on Syria last month suggest that a large building has been completely removed.
US research group, the Institute for Science and International Security, obtained and analysed the images.
The industrial-style building may have been a nuclear reactor under construction, says the ISIS.
A BBC correspondent says the images are not conclusive. Nor is it certain that they show the site hit by Israeli jets.
The Israeli strike has been shrouded in mystery and speculation.
Originally Israel did not even admit that the 6 September raid had been carried out, and its military censor ordered a complete blackout on information.
But Syria said Israeli warplanes violated its airspace in what it called a "hostile act", and Israel eventually acknowledged the mission some four weeks later.
Intelligence sources hinted at a possible link with North Korea's nuclear programme.
On Wednesday the Institute for Science and International Security (ISIS), an independent organisation, released satellite images pre-dating the attack, of a facility in northern Syria that it believes was the target.
They showed both a large industrial building and a pumping station near the Euphrates river.
SEQUENCE OF EVENTS
6 Sept: Syria says air defences fired at Israeli jets, which "dropped some ammunition without causing any material damage"
Week one: Israel says nothing; US officials say Israel struck an unspecified target; one US source hints at links to North Korea
Week two: N Korea denies any link to Syria; Israeli opposition leader Binyamin Netanyahu admits Israel made strike
Week three: Syrian president says a military construction site was hit and speaks of "retaliation"; Israel confirms strike on "military target"
The ISIS said the building bore a resemblance to the Yongbyon nuclear facility in North Korea.
"The length of the outer walls of the structures are approximately the same," the institute said in its analysis.
"From the image, the Syrian building is similar in shape to the North Korean reactor building, but the Syrian building is not far enough along in its construction to make a definitive comparison," it said.
The ISIS has now produced a more recent image of the same site taken on 24 October, more than six weeks after the alleged air attack.
The image appears to show that the building has been completely removed and the ground scraped clean.
Syria has consistently denied any plans to build a nuclear reactor, and its ambassador to the UN rejected the ISIS's analysis, saying the building was "only a centre for research for the desert areas, arid and desert areas in Syria".
"The main point is that the Israeli fighter jet violated the airspace of a member state of the United Nations. This is the only fact that we should focus on," Bashar Jaafari said.
"The Israelis have undertaken a provocative action and they should assume the consequences."
North Korea has adamantly denied that it was involved in helping Syria build any kind of nuclear facility.
The images, says the BBC's diplomatic correspondent Jonathan Marcus, are far from conclusive.
But they suggest that, for whatever reason, the Syrian authorities have gone to great lengths to remove any trace of the building apparently targeted in the strike, our correspondent adds.
The ISIS argues that "dismantling and removing the building at such a rapid pace dramatically complicates any inspection of the facilities and suggests that Syria may be trying to hide what was there".
The report also raises the question of whether Syria might be in breach of its safeguards agreement with the International Atomic Energy Agency, our correspondent says.
Under that, it would have an obligation to notify the UN's nuclear watchdog of any plans to construct a new nuclear facility.
While much about the Israeli air strike on Syria is a mystery, it is believed that in the early hours of 6 September a number of Israeli jets appeared to enter Syrian airspace from the Mediterranean Sea.
Later, unidentified drop tanks, which may have contained fuel from the planes, were found on Turkish soil near the Syrian border, indicating a possible exit route.
Witnesses said the Israeli jets had been engaged by Syrian air defences in Tall al-Abyad, north of Raqqa and near the border with Turkey.