NASA "very much wants to find signs of life in the universe," said associate administrator Amit Kshatriya, but the interstellar object called 3I/ATLAS was not it.
The US space agency released fresh images on Wednesday of the object, which zipped past Mars last month.
"I think it's important that we talk about that. This object is a comet," Kshatriya said. "It looks and behaves like a comet. And all evidence points to it being a comet."
Alien speculation fueled by the US government shutdown
The speculation that it was not a comet came during the US government shutdown, which prevented NASA from responding at the time.
Despite the fuzzy images, the agency says it has studied 3I/ATLAS using more than a dozen scientific platforms, including the Hubble and James Webb space telescopes and satellites orbiting Mars. The European Space Agency's two satellites around Mars also made observations.
"We were quick to be able to say, 'Yup, it definitely behaves like a comet.' We certainly haven't seen any technosignatures or anything from it that would lead us to believe it was anything other than a comet," Nicola Fox, associate administrator of NASA's Science Mission Directorate, said.
"It was great that the world wondered along with us," she added.
3I/Atlas on its way out of the solar system
Named for the telescope in Chile that first spotted it in July, the comet is believed to be even older than our solar system.
"It gives me goose bumps to think about," NASA scientist Tom Statler said, calling 3I/ATLAS "a window into the deep past and so deep in the past that it predates even the formation of our Earth and our sun."
Its origins remain unclear, but it is on its way out of the solar system. The closest the comet will come to Earth is 167 million miles (269 million kilometers) in mid-December.
NASA says the comet is visible from Earth in the predawn sky using binoculars or a telescope.
