As it turned out, authorities found out that the U.S. Bank Tower, the highest skyscraper downtown, was a potential target that day.
When Navy SEALs raided Osama bin Laden's compound earlier this year, they found a journal where the al Qaeda leader wrote about his desire to attack Los Angeles in the near future.
"You start wondering, how safe is downtown? How safe am I going to be in a building that's 20 stories tall?" said Cmdr. Blake Chow, who runs the Los Angeles Police Department's counterterrorism bureau. "for the last 10 years, there's been this refinement of bringing people together and really working together to really keep the Southland safe."
One of the biggest communication changes is the construction of a building downtown that houses the city's emergency response center.
In 2001, it was a dirt lot. Today, it's where the LAPD would manage the city's response in the event of an attack.
Agents can monitor cameras all over the city and coordinate a local, state and federal response from a central location.
"We're able to get information the FBI may be getting on the East Coast and synthesizing it and pushing it out and getting it to us very, very quickly," Chow said.
Another big change involves private security.
Brady Metcalfe of Universal Protection Services, who oversees 1,400 security agents in 70 buildings downtown.
"Private security works very closely and collaborates with law enforcement in a way that wasn't seen before," Metcalfe said.
Another major difference is the creation of RPIX, a computer network allowing the LAPD and private companies to share leads.
"It's similar to a social network, except it's strictly for sharing counterterrorism or major crime," Metcalfe.
"All the buildings are together, we all work together in case we have something or they have something we all share it," said Pete Morales, assistant security director for Universal Protection Services.
And there's a new way for the public to share suspicious information. It's called iWATCH. By calling (877) A-THREAT or logging on to iwatchla.org, people can report a threat anonymously.
"The more information we get, the better off we're going to be because we can process that, we can learn from it and if necessary, we can put the things in place to react to it," Chow said.