"The suspect throws tear gas in the air, and as the tear gas appears he started shooting," said Lamar Lane, who was watching the midnight showing of the movie with his brother. "It was very hard to breathe. I told my brother to take cover. It took awhile. I started seeing flashes and screaming, I just saw blood and people yelling and a quick glimpse of the guy who had a gas mask on. I was pushed out. There was chaos, we started running."
One witness said she saw people dropping to the ground after the gunshots began.
"We were maybe 20 or 30 minutes into the movie and all you hear, first you smell smoke, everybody thought it was fireworks or something like that, and then you just see people dropping and the gunshots are constant," witness Christ Jones told ABC's Denver affiliate KMGH. "I heard at least 20 to 30 rounds within that minute or two."
A man who talked to a couple who was inside the theater told ABC News, "They got up and they started to run through the emergency exit, and that when she turned around, she said all she saw was the guy slowly making his way up the stairs and just firing at people, just picking random people.
"The gunshots continued to go on and on and then after we didn't hear anything ...we finally got up and there was people bleeding, there was people obviously may have been actually dead or anything, and we just ran up out of there, there was chaos everywhere."
Witnesses and victims were taken to Gateway High School for questioning.
Hundreds of police and FBI agents are involved in the investigation. A senior official who is monitoring the situation in Washington said that early guidance based on the early snapshot of this man's background indicated that this act does not appear to be linked to radical terrorism or anything related to Islamic terrorism.
Dr. Comilla Sasson, at the University of Colorado Hospital where many of the victims were taken, said they were operating on nine critical patients and have treated 22 in all. She called the hospital "an absolutely terrifying scene all night."
"The good news is that the 3-month-old has actually been discharged home and is in the care of their parents."
In a statement, President Obama said, "Michelle and I are shocked and saddened by the horrific and tragic shooting in Colorado. Federal and local law enforcement are still responding, and my administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time. We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded."
For continuing coverage on "Tragedy in Colorado: The Batman Massacre," tune in to "World News," "20/20" and "Nightline."
Matthew Mosk contributed to this report.