Mikhy Farrera Brochez, the American at the centre of Singapore's HIV registry leak, has been charged by US prosecutors with threatening to extort the Singapore Government.

The latest charges come on top of existing allegations that Brochez, 34, possessed and unlawfully transferred stolen identification documents.

Brochez appeared in court for a scheduled hearing on Wednesday, during which US Magistrate Judge Matthew Stinnett ruled to refer his cases to a grand jury.

The grand jury will review the evidence and decide whether or not to indict Brochez.

If it does, he will be put on trial.

In an affidavit, also filed on Wednesday, FBI special agent Chelsea Holliday, who handled the case, said there was probable cause to believe that Brochez had transmitted a communication containing a threat to injure the reputation of officials and agents of the Singapore Government.

If found guilty, Brochez can be jailed for up to two years, fined up to $250,000, or both.

He has also been accused of knowingly and unlawfully possessing and transferring the means of identification of other people in violation of the US Code, said Ms Holliday, who was present at Wednesday's hearing but was not called to testify.

US prosecutor Dmitriy Slavin told the court that this second violation could carry a maximum penalty of up to 15 years in jail, a fine of up to US$250,000 (S$337,000), or both, if Brochez had obtained anything valued at US$1,000 or more.

In her affidavit, Ms Holliday said that during her investigations, she had reviewed an e-mail Brochez sent to several Singapore officials and government agencies on Jan 22.

The e-mail contained several of Brochez's previous allegations and a demand that the Singapore Government investigate the crimes allegedly committed against him while he lived in Singapore.

It also contained several links to documents stored on Google Drive, which were links to the medical database showing HIV-positive individuals in Singapore, she said.

Recounting a phone call with Brochez on Feb 19, Ms Holliday said: "He explained to me that if I could convince the Government of Singapore to release his husband, Siang, he would turn over the database.

"But if the Singaporean Government did not release Siang, he would release the database to the public."

Brochez's partner, Ler Teck Siang, has already been charged in Singapore under the Official Secrets Act.

In an interview after his arrest on Feb 21, Brochez told Ms Holliday and her colleague that he had sent the database because he wanted to "clear his name and to hopefully get his husband off these false charges".

Brochez had threatened to injure the reputation of the Government of Singapore, its agencies and its officials by publishing the medical database, said Ms Holliday.

She added that his e-mails and Facebook posts were made "with the intent to extort from them things of value".

"Namely, Brochez intended to obtain an investigation of crimes allegedly committed against him in Singapore, the end of the HIV registry and the release of Siang from imprisonment," she said.

Dressed in a yellow prison jumpsuit, Brochez at times spoke in Spanish to court officials.

He was asked as a matter of procedure whether he had any mental health issues preventing him from understanding the court processes.

Brochez said he understood the processes of the court hearing, but claimed that he suffered from post-traumatic stress disorder as the result of "torture by officials of the Singapore Government".

His court-appointed lawyer, Mr Jay Oakley, told reporters after the hearing that Brochez was unlikely to be extradited to Singapore, though the topic had not come up.

"He's an American citizen; I would say that the odds are likely not. But I haven't crossed that bridge or had any discussions with the government or my client about that," he said.

Asked how Brochez was feeling, Mr Oakley replied: "He's facing federal criminal prosecution in the US, so clearly he's not real thrilled about it. We're going to prepare our defence and handle the charges as they need to be handled.

"The federal grand jury will meet over the next couple of weeks, maybe next month, and decide whether or not there's probable cause to return an indictment against him."

Brochez remains in remand and has been ordered by the court not to disclose any confidential information he obtained from Singapore.

He has another court hearing on March 4 on a separate charge of trespassing on his mother's property last year.