9:05 AM PT -- Rini initially refused to provide fingerprints when he talked to investigators who met him at Cincinnati Children's Hospital, but authorities say he absolutely claimed to be 14-year-old Timmothy Pitzen. However, he agreed to the DNA testing.

When officers confronted him with the results, which proved he's NOT Timmothy, he immediately copped to being a phony. According to the U.S. Attorney, Rini said he'd seen Timmothy's story on an episode of "20/20" ... and cooked up the idea to impersonate him in order to get away from his own family.

He said, "He wished he had a father like Timmothy's because if he went missing, his father would just keep drinking."

And there's this ... the FBI says Rini's done this kind of thing twice before -- pretending to be a victim of juvenile sex trafficking. He was exposed both times through fingerprints.

The U.S. Attorney says if Rini's convicted this time he faces up to 8 years in prison.

8:51 AM PT -- Authorities have just announced that Rini has been charged by the FBI for making false statements to a federal agent.

The FBI released a statement, saying, "False reports like this can be painful to the families of missing children and also divert law enforcement resources in order to investigate these untruthful claims."

They continue, "Law enforcement takes dishonest reports very seriously, and we caution that people making false claims can and will face criminal penalties."

8:21 AM PT -- Federal and local law enforcement will be providing an update of the situation at 8:30 AM PT. We will be live streaming.

7:05 AM PT -- It appears the relevant law for which Rini's being held prohibits people from engaging in a "trick or scheme" to falsify, conceal or cover up a lie.

7:00 AM PT -- It appears Rini's accused of making false statements to law enforcement. Since the FBI conducted the DNA testing ... he most likely lied to FBI agents, and that's why the feds will be taking him into custody.

The guy cops say posed as missing child Timmothy Pitzen is now getting all the attention he wants ... in jail.

Brian Michael Rini was busted in Hamilton County, Ohio late Thursday night, and his mug shot raises a lot of questions. The 23-year-old who posed as Timmothy -- who would now be 14 years old -- is rocking a 5:00 shadow!

The case just gets more and more bizarre.

ICYMI ... someone called 911 on Wednesday to say Rini was wandering around and claiming to be Timmothy, who went missing 7 years ago -- a case that drew national attention, and was even recently featured on the A&E show, "Live PD." Local cops and the FBI responded and picked up Rini ... but immediately did DNA testing to confirm if he was really Timmothy.

On Thursday, the test came back negative, and law enforcement knew they had an impostor. It's still unclear what Rini's motive was and what he's being charged with for pretending to be Timmothy -- but we know the U.S. Attorney's Office is on the case. So, he's going to be charged federally.

He was detained Thursday night in Ohio, and the feds have scheduled a news conference for 8:30 AM PT ... which TMZ will live stream in this post.

Rini, as it turns out, has a lengthy rap sheet. He just got out of jail a month ago after doing 18 months for vandalism and burglary. He's also been convicted of unauthorized use of a vehicle.