Keyes had discussed "seven or eight other victims" before taking his own life in December in an Alaskan jail, Special Agent Jolene Goeden of the FBI Anchorage division said in a statement.
Keyes, 34, has been linked to 11 killings from 2001 to March 2012, the FBI said Monday.
During interrogations by the FBI after his arrest, Keyes "gave us a number of clues," Goeden said. "He talked openly about some of the homicides, but much of what he said only hinted at the things he had done. So we are trying to get information out there about what he did tell us."
Anyone who might have information about Keyes was asked to call the FBI at 1-800-CALL-FBI.
Agents are now trying to determine if Keyes' killing spree continued while he was out of the country, CNN reported. He is known to have been in Belize, Canada and Mexico.
"Keyes traveled internationally and it is unknown if he committed any homicides while outside of the United States," the FBI said in a statement that asked for information on his movements.
The four-page note Keyes, 34, left gave few details about his crimes and expressed no remorse. But the FBI said an analysis of statements he made after his arrest led them to increase the death toll.
Keyes, who studied other serial killers, is believed to have committed crimes across the country, including a couple in Vermont in 2011. His last known victim, Samantha Koenig, a barista in an Anchorage coffee house, who died in 2012.
Keyes, who had been using Koenig's debit card, was arrested in Texas and extradited to Anchorage. After his death, investigators found two caches he had created with equipment to kill and to dispose of bodies, one in Alaska and the other in the Adirondack region of New York.