WASHINGTON — If President Obama hoped that the danger of chemical warfare in the Middle East receded when Syria gave up tons of poison gas, mounting evidence that toxic weapons remain in the strife-torn country could once again force him to decide just how far he is willing to go to enforce his famous “red line.”

The discovery of traces of ricin and sarin in Syria, combined with the use of chlorine as a makeshift weapon in the country’s grinding civil war, undercut what Mr. Obama had viewed as a signal triumph of his foreign policy, the destruction of President Bashar al-Assad’s chemical arsenal.

But Mr. Obama appears no more eager to use military force against Mr. Assad’s government today than he was in 2013 when he abruptly called off a threatened airstrike in exchange for a Russian-brokered agreement in which Syria voluntarily gave up its chemical weapons. Instead, the Obama administration responded to reports of violations this time by seeking renewed assistance from Russia and exploring a new United Nations Security Council resolution addressing Syria’s continued use of chemicals as weapons.

“You’re dealing with a regime that is not very credible on weapons of mass destruction programs,” said Robert Ford, the Obama administration’s former ambassador to Syria. “No one should be surprised the regime didn’t declare all of its facilities. But the bad news in all of this is the regime is using chemical weapons regularly — even if not sarin gas now, they’re using chlorine gas regularly and they are not deterred from doing so.”