In July 2012, senior U.S. intelligence officials drove to the Capitol to secretly brief top lawmakers on the first indications that the Syrian government had used chemical weapons against its own people.

The classified reports about a small-scale attack weren’t definitive, according to U.S. officials who were privy to them. It was still a month before President Obama warned that the use of chemical weapons would cross a “red line” and “change his calculus” about taking action in Syria.

But it was the beginning of a stream of intelligence documenting what U.S. officials say was a yearlong escalation in the use of the banned weapons by the government of President Bashar Assad, a far more extensive record of the incidents than previously known. The Obama administration did not publicly acknowledge the attacks for months, and declared in April that it believed Syria had used chemical weapons…

Administration officials say the evidence for previous chemical attacks wasn’t as compelling, and critics acknowledge it would have been even harder to make the case for a military response to more limited use of the banned weapons. But some current and former officials say the slow response by the White House raises questions about whether earlier, clearer warnings by Obama — and perhaps limited actions such as providing sophisticated weapons to Syrian rebels — could have deterred last month’s attack in Damascus suburbs.