KALAMAZOO, Mich. (AP) — A lawsuit against Uber filed in the name of a driver accused of gunning down six people in between stops to pick up fares is a hoax, authorities said Thursday.

Kalamazoo County Undersheriff Paul Matyas said he saw reports Wednesday about a 1½-page handwritten lawsuit by Jason Dalton seeking $10 million from the ride-hailing service and became suspicious because the envelope it reportedly was mailed in had a Philadelphia postmark.

An investigator spoke with Dalton, who is jailed, and he denied filing the lawsuit, Matyas said. Dalton also said he didn’t authorize anyone to file a lawsuit on his behalf and he didn’t know who would have done so.

The sheriff’s department compared the writing in the lawsuit with a sample of Dalton’s writing and it didn’t match, Matyas said. It also wasn’t in an envelope typically used jail inmates, he said.

The lawsuit was listed in federal electronic records as being filed by Dalton in U.S. District Court in Detroit and was given a case number.

Asked whether his department was trying to find out who is responsible for the hoax, Matyas said his office referred the matter to the FBI, since the lawsuit was filed in federal court.

Jill Washburn, a spokeswoman for the FBI’s Detroit office, said the case is “considered a civil matter as far as we’re concerned” and the agency doesn’t plan to investigate it.

Dalton’s attorney, Eusebio Solis, didn’t respond to an email.

Detroit federal court spokesman Rod Hansen said the filing went through the typical steps: It was processed by the clerk’s office and assigned both a magistrate judge and judge. Hansen said such letters from jail or prison inmates are common.

“There was no reason for us to believe it was a hoax or non-hoax,” he said. “At some point along the way, those procedures are going to catch the fact that this person isn’t who they say they are.”

Hansen added the court isn’t an investigative agency but would cooperate with any investigation into the matter.

Dalton is charged with murder and attempted murder in the killing of six people and wounding of two others over a span of several hours on Feb. 20. Authorities say that in between picking up Uber fares in Kalamazoo, Dalton opened fire on people at three locations, and that he didn’t know any of the victims.

According to police, Dalton told investigators that “a devil figure” on Uber’s app was controlling him.