Rarely does President George W. Bush bite his lip when discussing “axis of evil” states such as Syria and North Korea.

But for more than two months Mr Bush has done just that when asked about a September 6 Israeli strike on a mysterious Syrian site. So has every other US administration official speaking on the record, as well as the Israeli government itself, fuelling a continuing controversy about the incident.

“This is not my first rodeo,” Mr Bush said at a press conference last month after being asked about the incident at least six times. “And I know where you’re trying to get me to comment. I’m not going to comment on it, one way or the other.”

This uncharacteristically taciturn approach has angered some of the president’s Republican supporters, amid a series of newspaper reports that the Syrian site was a nuclear reactor built with North Korean help. But as the weeks since the Israeli raid pass, more questions than answers have surfaced about what precisely the Israelis hit.

The Republican anger has been displayed by legislators such as Peter Hoekstra and Ileana Ros-Lehtinen, respectively the senior Republican members of the House of Representatives committees on intelligence and foreign affairs. Among a select handful of congressmen briefed on the incident by Michael McConnell, the US’s director of national intelligence, they did not respond to the briefing as intended, calling instead for the administration to come clean publicly about what had occurred.

“Until Congress is fully briefed it would be imprudent for the administration to move forward with agreements with state proliferators,” they said in the Wall Street Journal, in reference to the six-party talks on Korea’s nuclear programme.

John Bolton, formerly Mr Bush’s ambassador to the UN, argues that the administration is unduly preoccupied with reaching a deal in those negotiations and with the forthcoming international conference in Annapolis, Maryland, on the Israel-Palestinian conflict, to which Syria is likely to be invited.

“There’s a growing suspicion that the veil of secrecy about Syria doesn’t have so much to do with intelligence as with protecting the six-party talks and the Annapolis conference,” Mr Bolton says. “Israeli government officials have told me that Secretary [Condoleezza] Rice told them in no uncertain terms that the information [about the attack] better not come from them.”

David Albright, a respected former United Nations weapons inspector, has used satellite imagery to argue that the Israelis probably attacked an installation on the Euphrates river at Dawar az Zawr, eastern Syria, which was similar in shape to North Korea’s Yongbyon reactor.

“I believe pretty strongly that Israel thinks it attacked a nuclear reactor and that the US didn’t say they were wrong,” he says, adding that his suspicions were heightened by the recent addition of a pump house that could supply water for a reactor, and further images that showed Syria razed the site after the Israelis struck.

“The recent photo evidence showing the Syrians clearing the sites indicated that they are clearly trying to hide something,” says Joe Cirincione, a non proliferation expert at the Centre for American Progress. He says that the evidence so far “tilts towards the theory that it was a reactor in Syria but does not establish it”.

But he adds: “There’s no evidence it was an imminent threat,” arguing that if there was a reactor, it was probably years from producing plutonium and that Syria lacks a reprocessing facility to turn its spent reactor fuel into plutonium.

The International Atomic Energy Agency, the UN’s Vienna-based nuclear watchdog, is also not convinced the site was a reactor.

Diplomats in Vienna warn against relying on satellite photos, saying that it is quite possible that the installation was no more than the adjunct of a chemical facility or a shed used to make agricultural equipment. They add that they were surprised that the site lacked a security fence, an unusual omission in a top secret facility.

“I am amazed that people can say they know the function of an installation just because they see its dimensions,” adds Jeffrey Lewis, an arms control expert at the New America Foundation. “It’s a box on a river.” He adds that, far from being in a remote site, the Dawar az Zawr facility is just over a mile from a well-known tourist site in Halabiya, from which rafting trips down the Euphrates can be organised.

Meanwhile, President Bashar al-Assad says the Israeli aircraft hit a disused military building with “nothing in it”.

The mystery may not continue indefinitely: under the talks framework, North Korea is due to give an account of its nuclear activities by the end of the year. And so far, despite Mr Bolton’s concerns, the events in Syria have not interrupted those negotiations.

“The opponents of the six-party talks wanted to use this strike to kill this deal,” said Mr Cirincione. “But the president isn’t having it.”