An investigating judge has opened a probe into an alleged "terrorist attempted murder" four days after 46-year-old Jurgen Conings disappeared, leaving behind "troubling" letters threatening coronavirus experts.

The search for the serviceman, which has been joined by specialist police units from four countries and troops backed with armoured cars and helicopters, has focused around a national park in northeastern Belgium, near the Dutch border.

But four days later, officers have lifted a cordon around the Hoge Kempen park, near where the suspect's abandoned car was found earlier in the week with anti-tank rocket launchers inside. He is thought to have taken more weapons with him and gone on the run.

Officials have said Conings -- dubbed the "Belgian Rambo" in the press -- was on a monitoring list of suspected far-right extremists and had been stripped of a high-level security clearance and disciplined over racist and violent social media posts.

Now, he is also the subject of an investigation by anti-terror prosecutors. Potential targets -- such as renowned virologist Mark Van Ranst -- have been moved to places of safety under police protection.

"The work of the investigation is continuing," the spokesman for the federal prosecutor's office Eric Van Duyse said. "An investigating judge has been assigned on a count of attempted terrorist assassination and possession of weapons in a terrorist context."

Justice Minister Vincent Van Quickenborne said the suspect, who disappeared on Monday, had spent more than two hours that evening near the home of one of his potential targets. Broadcaster VRT reported that the target was Van Ranst, but the minister did not confirm this.

Van Ranst has become a media figure as an expert commentator during the coronavirus crisis and a target of attacks from conspiracy theorists and the Flemish far-right.

- Acute threat -

The case has drawn international attention and embarrassed the Belgian government, forced to explain why a soldier with extremist views was allowed to retain access to military weapons, especially when conspiracy theories swirling around the coronavirus crises have stirred up far-right circles.

"There was a failure in procedures," Defence Minister Ludivine Dedonder told public broadcaster RTBF.

She vowed to seek a change in the law to allow a more thorough vetting of any soldier granted access to sensitive sites and weapons -- like the rocket launchers found in Conings' abandoned car.

Separately, Prime Minister Alexander de Croo has ordered a report into extremism within the ranks of Belgium's police, armed forces and customs service and will present an "action plan" next month.

Conings is assumed to present an "acute threat" of violence and is thought to have taken several more weapons from the base where he served as an instructor.

Dedonder said Conings had faced a disciplinary procedure but, as an instructor charged with preparing recruits for overseas missions, he still had access to a dangerous arsenal of military weapons, she admitted.