Nigel Duara / The Associated Press

PORTLAND — An inmate at a county jail is suspected of mailing a harmless powder he claimed was anthrax to the attorney prosecuting his case, one of a series of powder-filled letters sent around Portland in late April and early May.

Investigators were first tipped to the suspect by some old-fashioned police work: They checked the return address on the envelope.

In a search warrant seeking the inmate’s DNA, an FBI agent said Assistant U.S. Attorney Steven Peifer received the threatening letter with a return address of “Barnett, Andrew.”

Barnett allegedly encouraged Peifer to snort the white powder included in the letter, telling Peifer in the letter, “I want you gone!!!!”

Peifer has recused himself from Barnett’s case. Barnett has not been accused in connection with any of the other mailings, which went to a mall, a downtown hotel and the airport.

Staff at the Multnomah County Detention Center, where Barnett is being held, were instructed by a judge on May 1 to forward all letters from Barnett to his own defense attorney because of his alleged propensity to send threats in the mail.

Some of the letters were received more than one week after the judge handed down the order. On May 10, U.S. District Court Judge Ancer Haggerty ruled that Barnett wasn’t permitted to send mail at all.

“Despite (the May 1) order, defendant has persisted in sending mail to the court, and it is believed, in sending threatening mail to certain public premises,” Haggerty wrote.

It’s unclear if Barnett managed to mail any of his letters from jail after the judge’s order that all of his mail go to his attorney. A Multnomah County Sheriff’s Office spokesman declined to comment, citing the ongoing investigation.

Barnett’s attorney, J. Henry Langer, could not be reached on Friday for comment. Barnett is also under indictment for threatening letters authorities said he sent to the Washington County sheriff and sheriff’s deputies in January 2008.