Just before today's anniversary, federal authorities concluded that the smoldering wreckage of the collapsed twin towers was responsible for the cancers of many of the volunteers who worked on the site or lived near the site in the months after the attack. Fifty cancers were added to a list of medical conditions for which emergency workers who were sent to Ground Zero and became sick can now receive compensation for treatment.
On Monday, officials announced a deal to resume construction on a 9/11 museum in lower Manhattan. The deal would put the Port Authority of New York and New Jersey in control of the museum.
Vice President Biden spoke at the site in Pennsylvania where United flight 93 crashed on 9/11 when passengers confronted a group of terrorists intent on flying the plane into the White House.
"We wish we weren't here. We wish we didn't have to be here. We wish we didn't have to commemorate any of this," Biden said.
ABC News' Aaron Katersky contributed to this report.