Syrian dictator Bashar Assad must turn over his chemical weapons in order to avoid military reprisal by the United States, but President Obama's spokesman can't say if the regime will retain biological weapons.

"I'll have to defer that to the experts who are negotiating this, perhaps the State Department or others — the team that's going with Secretary Kerry," White House press secretary Jay Carney said during the Wednesday briefing. "What is obviously directly of concern here was the use of chemical weapons by the Assad regime. But I think that's a question that's worth following up on."

Carney was replying to reporter who asked, "should this agreement also include Assad turning over his biological weapons?"

The comment most often associated with Obama's red line pertains only to chemical weapons. "We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized," he said. "That would change my calculus. That would change my equation.”

Moments earlier, though, Obama also mentioned Assad's biological weapons caches. “[T]he point that you made about chemical and biological weapons is critical," he told reporters. That’s an issue that doesn’t just concern Syria; it concerns our close allies in the region, including Israel. It concerns us. We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people.

Note: NBC's Chuck Todd didn't mention the biological weapons in his question. "Mr. President, could you update us on your latest thinking of where you think things are in Syria, and in particular, whether you envision using U.S. military, if simply for nothing else, the safe keeping of the chemical weapons, and if you're confident that the chemical weapons are safe?" he asked.

So, biological weapons were certainly on Obama's mind when he staked out the red line.