A law enforcement official familiar with the emergency response on Monday, however, said there did not seem to be "a lack of resources" that morning, noting that early on there was still "the possibility that there could be other targets as part of a complex attack."
One more response team at the Navy Yard "probably would not have changed the outcome," the official said.
Nevertheless, a shootout ensued once responders found Alexis inside Building 197.
Comey said investigators have found no indications that others are connected to Monday's attack, and they were still struggling to identify a specific motive.
Still, Comey acknowledged Alexis' deteriorating mental health likely played a role.
"There are indications this was a person with mental health troubles, and we're trying to understand that," Comey said.
ABC News has learned Alexis etched the phrases "better off this way" and "my elf weapon" on the stock of the shotgun, which he purchased from a Virginia gun store only days before the attack. Investigators are trying to figure out the meaning of those phrases, especially whether "better off this way" was a signal Alexis expected to be killed by police.
In the days since the attack, details have emerged suggesting Alexis was increasingly becoming paranoid, including recent rantings to police in Newport, R.I., that he was hearing voices in his head and felt microwave machines bombarding his body.
Local police alerted Newport Naval Station authorities about that bizarre story, but the information was never passed up the chain of command, the Navy confirmed Wednesday.
Alexis kept his job as a civilian contractor with the Navy, holding a secret security clearance.