A US man has lost an appeal over his refusal to decrypt hard drives seized during an investigation regarding child sexual abuse images.

The man, who has not been named in court documents, has been held in custody for nearly 18 months.

No criminal charges have been filed against him.

The man’s lawyer has argued that his client is protected by the Fifth Amendment, which protects US citizens from incriminating themselves.

A police search in 2015 at the suspect’s home recovered an Apple iPhone 5S, Mac Pro computer and two external hard drives.

Authorities allege that they found evidence suggesting that child sexual abuse images had been accessed with the devices after they were able to decrypt the Mac Pro.

Prosecutors also say that images of a six-year-old girl that “focused” on her genitals were found on his iPhone 6 Plus, which had been seized separately and which the suspect unlocked during a forensic examination.

The external hard drives remain inaccessible, however, and the suspect has been held in contempt of court – and remanded in custody – since late 2015.

At one hearing, court documents say the suspect claimed he could not remember the password to unlock the drives.

‘Disappointed by ruling’

“The government has provided evidence to show both that files exist on the encrypted portions of the devices and that [the suspect] can access them,” wrote the circuit judges, rejecting the appeal.

They added that they disagreed the Fifth Amendment was grounds for a successful appeal in this instance.

“We are disappointed in the ruling and [are] studying the decision to determine what further review it may be appropriate to seek,” said Keith Donoghue, a federal defender representing the suspect.

“The fact remains that the government has not brought charges and our client has now been in custody for nearly 18 months based on his assertion of his constitutional right against self-incrimination.”

The US Supreme Court has never ruled on the issue of whether suspects can cite constitutional protection when refusing to unlock a smartphone or decrypt a computer drive.

Taking the Fifth

However, it is becoming an increasingly common discussion in American courts.

Last year, in a separate case, a Florida court said a suspected voyeur could be compelled to reveal his iPhone passcode to investigators.

“I think they got it wrong,” Mark Rumold, a senior staff attorney for the Electronic Frontier Foundation, told the BBC at the time.

He added that he believed there were “sound constitutional reasons” for preventing the state from compelling information from a defendant’s mind.