Most of the mass shootings in recent years have been linked to terrorists, so the possibility arose immediately when Stephen Paddock unleashed a barrage of gunfire at a crowd at a Las Vegas concert in October, killing 58 and injuring another 500.
Since then, authorities have stated they see no link to terrorism, although they have not provided any conclusions regarding a motive.
This week, authorities said they can't do that yet, because there still are suspects being investigated.
But one member of Congress now is raising the issue, citing what he describes as "credible evidence of a possible terrorist nexus" in the Las Vegas case.
The Hill reports Rep. Scott Perry, R-Pa., said on Fox News Thursday: "I smell a rat like a lot of Americans. Nothing's adding up."
Perry said it's been "four months, as you said, the man's dead, they said he's a lone gunman, lone shooter, yet we can't get the autopsy results."
"But even more troubling than that," he said, "I've been made aware of what I believe to be credible evidence, credible information, regarding terrorist infiltration through the southern border regarding this incident."
Without providing details, Perry explained ISIS in Syria and Iraq previously claimed it had plans to attack Las Vegas, and then terrorists took credit when it happened.
The FBI said at the time that experts were puzzled by ISIS' claim of credit.
Catherine Lombardo, a lawyer for the victims of the Paddock attack, also told Tucker Carlson on his Fox News show that there's been "no evidence of a terrorist attack."
"I would ask, with all due respect congressman, unless you have specific evidence to back that up, it seems a bit irresponsible to make that allegation. If you do have any evidence of that, I'm asking you right now."
But he declined to respond to her challenge.
"I'm just telling you, I have received what I feel to be … credible evidence of a possible terrorist nexus. We're going to have to wait until that situation develops," he said.
The attack by Paddock, who ended up dead, was the biggest mass shooting in recent history.
And Perry noted: "ISIS twice before the attack, ISIS warned the United States that they would attack Las Vegas I think in June and August and then after the attack claimed responsibility four times. Meanwhile the local law enforcement investigative services are telling us there is no terrorist connection lone gunman, again, something is not adding up."
There's also been no explanation of where and how Paddock would have obtained the knowledge needed to stage such an effective attack.
Several left-wing organizations, without offering explanations for the questions he raises, described the congressman as a conspiracist.