NEW YORK — Flight attendant Betty Ong couldn’t tell exactly what was happening in the cockpit of American Airlines Flight 11, but it was clear to her that there was trouble.

“I don’t know, but I think we’re getting hijacked,” she said in a phone call to an American Airlines reservation desk at 8:19 a.m. on Sept. 11, 2001.

The audio recording of that call — her relaying that two other employees had been stabbed, that they couldn’t get into the cockpit and didn’t know who was in there, that someone had sprayed something into the air, the long stretches of silence on the other end of the phone as her listeners seemingly struggled to fully absorb what they were being told — is part of an online timeline that attempts to give a sense of order to that most chaotic of days.

The timeline, put together by the National September 11 Memorial & Museum and launched Wednesday, incorporates audio recordings from phone calls on that day, oral histories from survivors and eyewitnesses and graphic photographs and video snippets arranged in chronological order. Viewers can use social media including Facebook and Twitter as well as e-mail to share links to the site and to particular photos and videos.

The timeline starts at 5:45 a.m., with photographs of hijackers Mohammed Atta and Abdulaziz al-Omari passing through airport security in Maine for a flight to Boston, where they would board Flight 11. It ends at 8:30 p.m., with President George W. Bush addressing the nation.

Along the way, it outlines the departures of all four fatal flights and shows images of their passenger manifests, video and photos of the World Trade Center’s north and south towers after they were hit and heartbreaking moments such as when United Airlines Flight 175 passenger Brian Sweeney left a voice mail for his wife, Julie Sweeney.

“Jules, it’s Brian. Listen, I’m on an airplane that’s been hijacked. If things don’t go well, it’s not looking good, I just want you to know I absolutely love you,” he said.

The timeline doesn’t shy away from the starkest images of the day. In one video of the collapse of the south tower, an onlooker can be heard saying, “Oh, my God!” repeatedly as the tower falls. A video of the fall of the north tower carries a warning of mature language, as people can be heard screaming and cursing.

The president of the museum, Joe Daniels, said the project’s organizers were sensitive to the nature of what they were presenting and took steps such as leaving it up to viewers as to whether they wanted to take closer looks at specific photographs and videos or listen to particular bits of audio.