From ABC News:

Former President Bill Clinton took the phrase "on-message" to a new level in Denver when hecklers began shouting about how September 11th was a conspiracy theory that needed further investigation.

Clinton had just begun his speech to the crowd, when a man towards the front of the crowd began shouting about re-opening the 9/11 investigation and the war in Iraq.
When the crowd tried to boo the man into silence, Clinton continued: "Let him talk."

The heckler's exact wording wasn't quite audible from the press risers, but the tone was apparent to everyone in the arena, including Clinton.

"Are you one of those 'it was an inside job' guys?" the former president asked. Continuing, "Hillary agrees we should end the war, but we have heard from you now, now you hear from me. I let you be rude and interrupt me screaming at the top of your lungs. Nine-eleven was NOT an inside job, it was an Osama Bin Laden job with 19 people from Saudi Arabia, they murdered 3000 Americans and others foreigners including Muslims and we look like idiots, that the people who murdered our fellow citizens did it, when they are continuing to murder other people around the world. So we heard from you, you go away," Clinton said to the cheering crowd.

Undeterred, the heckler and his 9/11-sign waving cohorts started up a few minutes later, and Clinton decided to use this opportunity to promote his wife's record in the Senate.

"One of the reasons you ought to be for Hillary for President, since this has been brought up, is that she helped pass bi-partisan legislation in the Senate to help take care of all those firefighters and police officers who got sick because of 9/11. You talk to the victims of 9/11, the people who had a dozen surgeries just so that they could go back and their bodies could work again, and you ask them if they think it was an inside job, or whether they are glad they had a Senator like Hillary Clinton who gave them a chance to they could stand up and start their lives over again," responded Clinton.