Why didn't president FDR act after the attack on the USS Greer?

The official version of events put out by the Roosevelt Administration was that the USS Greer had been fired upon by a German submarine. The Germans disputed this account and stated instead that it was they who'd been attacked with depth bombs by the Americans. They further accused the government of deliberately trying to provoke the Germans into conflict in order to justify the United States' entry into the war. President Roosevelt responded by authorizing the US Navy to shoot on sight any German submarine, in any waters, deemed a threat to the United States.
Yet FDR didn't use the attack on the Greer as a pretext for going to war. Isolationist sentiment was still pretty high in the United States, and Roosevelt understood that only a major full-scale attack on America would be enough to change the predominant attitude. Just such an attack duly came with the Japanese assault on Pearl Harbor in December, 1941, just over three months after the attack on the Greer.