The Justice Department is maintaining tight secrecy around its case against a 36-year-old white supremacist accused of planting a backpack bomb on the route of a Martin Luther King Jr. Day unity parade in Spokane, Wash.

Kevin William Harpham, who has past ties to the neo-Nazi National Alliance and made more than 1,000 postings on racist Internet sites, was indicted Tuesday, 13 days after his arrest by the FBI’s Hostage Rescue Team and other agents.

If convicted of attempting to use a “weapon of mass destruction” and knowingly “receiving and possessing” an illegal explosive device, Harpham was told at his arraignment Wednesday that he could be sent to prison for life.

But just how an FBI terrorism task force and Justice Department prosecutors built their case again the former U.S. Army soldier remains a secret. There have been no other arrests and Harpham wasn’t charged with conspiracy, which would have been a clue that FBI agents were still looking for one or more additional suspects.

During his brief arraignment in Spokane before U.S. Magistrate Judge Cynthia Imbrogno, a team of deputy U.S. marshals guarded Harpham, who was kept in ankle chains.

“May I enter ‘not guilty’ pleas on your behalf?’’ the judge asked Harpham, dressed in a tan Spokane County Jail jumpsuit, as he stood beside his attorney, federal defender Roger Peven.

“Yes, please, your honor,’’ Peven responded.

“Not guilty,’’ Harpham added with a quick, firm voice.

An affidavit used to arrest Harpham on March 9 near his rural home in northeastern Washington state remained under seal Wednesday at the request of U.S. Attorney Mike Ormsby of the Eastern District of Washington. Ormsby has said the affidavit detailing the case against Harpham is being kept from the public to protect his right to a fair trial.

Prosecutors even took the unusual step of redacting the name of the “foreperson” of the 23-member federal grand jury that returned the two-count federal indictment on Tuesday. The grand jury heard federal agents and prosecutors outline their evidence against Harpham in a secret session, one day before a public probable cause hearing would have been held in open court before Judge Imbrogno.

Even Peven, Harpham’s federal defender, said after Wednesday’s arraignment that he still has no clue yet how investigators built their case against his client.

“It is somewhat unusual,’’ Peven said outside the courthouse when asked about the secrecy surrounding the case.

Now that Harpham has been indicted, Justice Department prosecutors are compelled under court “discovery” rules to begin turning over investigative files to the defense, even though public access to the information remains blocked.

Peven said he expected to receive shortly an affidavit of probable cause that outlines how Harpham was identified and then arrested. But, he said, it probably will be several days or weeks before he begins receiving FBI “302” reports detailing the investigation.

Because he knows so little about the investigation, Peven said he is still not ready to attempt to ask for a bond hearing — a move federal prosecutors likely would strongly oppose on the grounds that the defendant poses a risk to the community, if not a flight risk.

Asked to comment about the defendant’s ties to white supremacist groups, Peven said, “I really can’t at this point.”

The Southern Poverty Law Center (SPLC) has information showing that in 2004 Harpham was a member of the National Alliance, which for decades was the preeminent neo-Nazi organization in the United States. The group went into a steep decline after the 2002 death of its founder, and today is merely a shadow of its former self.

Research by SPLC also shows that Harpham posted more than 1,000 times on the Vanguard New Network (VNN), a leading racist and anti-Semitic site. Using the pseudonym “Joe Snuffy” in 2006, Harpham posted, among other things, “I can’t wait till the day I snap.”

Harpham’s father, Cecil “Bill” Harpham of Kettle Falls, Wash., told two Spokane televisions stations that his son was caring for him in the senior Harpham’s Stevens County home and couldn’t have traveled 60 miles to Spokane to plant the improvised explosive device. But he conceded that his son had racist views and might have built the bomb.

The device was found and tragedy averted at the last minute when day-laborers found it Jan. 17 in a backpack on a downtown Spokane street corner. Police then re-routed the “MLK Unity Day” parade — which had some 1,500 participants — and a bomb squad successfully disarmed the device without substantially damaging it.

It’s widely believed that the nearly intact backpack bomb — apparently intended to be detonated with a remote control device — provided a windfall of forensic evidence, possibly including DNA, fingerprints and product samples, during days of processing at the FBI lab in Quantico, Va.

The Seattle Times, quoting an unidentified source familiar with the investigation, said the FBI linked Harpham to the bomb through DNA and the purchases of bomb components, including a remote car starter and other electronics.

Meanwhile, what’s identified as an FBI “unclassified-law enforcement sensitive” report on the Spokane backpack bomb has anonymously been posted on the Internet.

“We assess with medium confidence that the [explosive device] was designed to fire directional fragmentation similar to a single shot shotgun with buckshot or cannon with a grapeshot round,” the report says. “We likewise assess that the device was viable and could have caused personal injury or death.”

The report added: “Although the use of a radio-controlled trigger suggests the bomb maker possesses a greater degree of knowledge and skill than the typical pipe bomb builder, the device incorporated components that are commonly available in the United States and relied on techniques published in open sources.”