A few days ago, a little-known Swedish scientist with a career devoted to studying lethal warfare agents paid a quiet visit to London. He was there to examine evidence that British officials believe shows that Syrian forces used chemical weapons against their own people.

Ake Sellstrom’s confidential mission marked the first stage in a fledgling U.N. investigation into claims that the nerve agent sarin was used in battles in at least three Syrian cities since December. The inquiry has once again thrust the United Nations into the center of a hunt for weapons of mass destruction.

For U.N. inspectors, the inquiry is reminiscent of the days when they scoured Iraq’s deserts and industrial parks more than a decade ago in pursuit of lethal stockpiles of chemical weapons that had long before been destroyed and nuclear facilities that no longer existed.

There are, to be sure, stark differences between Saddam Hussein’s Iraq and President Bashar Assad’s Syria. For one, the United States, which led the push for war in Iraq, appears reluctant to enter the war in Syria. For another, U.N. inspectors may never be allowed to enter Syria to examine the sites in question, making it extremely difficult to establish definitively whether chemical weapons were used and by whom.

But officials at U.N. headquarters also see the parallels and potential pitfalls between Iraq and Syria. Among them is a big-power rift between the United States and Russia and the reactivation of several veterans of the Iraq inspections, including Sellstrom.

As happened with Iraq, any findings by the U.N. team will fuel a debate about the wisdom of military intervention in Syria. Its conclusions also will test the reliability of Western intelligence agencies, particularly in the United States and Britain, whose flawed intelligence served as the basis for the U.S.-led invasion of Iraq in March 2003.

“The echoes of weapons inspections in Iraq are inescapable,” said Carne Ross, a former British diplomat who managed his government’s Iraq policy at the United Nations from 1997 to 2002.

“We were played, right?” said Patricia Lewis, an expert on biological, chemical and nuclear weapons at London’s Chatham House, suggesting that the abuses of intelligence in Iraq will cast a shadow over the findings on Syria. “We were badly burned by people coming out of Iraq telling us stuff that wasn’t true, either out of enthusiasm or malice or ego. Nobody outside the West believes the intelligence services of the West anymore.”

President Barack Obama has said repeatedly that any use of chemical weapons by Syria would cross a “red line,” prompting an unspecified response. But Obama, who opposed the Iraq War, has also underscored the limits of intelligence and the need for concrete evidence.

In a letter to key lawmakers last week, the White House said U.S. intelligence agencies believe the Syrian government is likely to have used chemical weapons on a small scale. But the letter said U.S. officials are seeking further proof and endorsed a “comprehensive United Nations investigation that can credibly evaluate the evidence and establish what took place.”

In late March, the Assad government asked the United Nations to investigate its claims that Syrian opposition fighters attacked government forces with chemical weapons in the town of Khan al Assel, near the city of Aleppo. Negotiations over the scope of the U.N. investigation stalled after U.N. Secretary General Ban Ki Moon accepted a subsequent request by Britain and France for a broader inquiry, which would also examine claims by opposition forces that Syrian authorities used chemical weapons.

Syria’s U.N. ambassador, Bashar al-Jaafari, said his government has proof that chemical weapons were used at Khan al Assel. “We have victims, we have bodies, we have wounded,” he said Tuesday at the U.N.

Jaafari said the Syrian government opposes the expanded U.N. mission, citing concern that it would mark the beginning of an open-ended investigation leading nowhere. “What happened in Iraq is still alive in our minds,” he said.

Experts say detecting the use of chemical weapons grows more difficult as time passes. If the U.N. team never gets into Syria, the job will be even harder.

Either way, the task will test Sellstrom’s technical and diplomatic skills as he seeks to stitch together the evidence — primarily soil samples, blood and hair from suspected victims, and witness testimony — to come up with conclusive answers.

He will be working with a much smaller team — about 15 chemists, munitions experts and doctors — than the one deployed to Iraq. In the coming weeks, the experts are expected to travel to key capitals to examine the findings of intelligence services and interview Syrian refugees who may have witnessed, or been exposed to, the use of chemical weapons.

Those who know the Swedish scientist say that he will follow the evidence where it leads and that he is unlikely to be intimidated or swayed either by Syrian combatants whose claims he is testing or the big powers that will be supplying him with evidence.

The Iraq inspectors reported to the Security Council, but Sellstrom and his team may have more latitude because they were appointed directly by Ban.

Still, insiders caution, Sellstrom risks being drawn into a major-power squabble over the course of the inspections, particularly if the team is unable to come up with a definitive conclusion.