In this grab taken from a footage provided by the Russian State Atomic Energy Corporation ROSATOM press service, people gather for the funerals of five Russian nuclear engineers killed by a rocket explosion in Sarov, the closed city, located 370 kilometers (230 miles) east of Moscow, Monday, Aug. 12, 2019. Thousands of people have attended the burial of five Russian nuclear engineers killed by an explosion during tests of a new rocket. The engineers, who died on Thursday, were laid to rest Monday in the city of Sarov that hosts Russia’s main nuclear weapons research center. (Russian State Atomic Energy Corporation ROSATOM via AP)

MOSCOW (AP) — Thousands of people attended the funerals Monday of five Russian nuclear engineers killed by an explosion as they tested a new rocket engine, a tragedy that fueled radiation fears and raised questions about a secretive weapons program.

The engineers, who died Thursday, were laid to rest Monday in Sarov, which hosts Russia’s main nuclear weapons research center, where they worked. Flags flew at half-staff in the city 370 kilometers (230 miles) east of Moscow that has been a base for Russia’s nuclear weapons program since the late 1940s. The coffins were displayed at Sarov’s main square before being driven to a cemetery.

The Defense Ministry initially reported the explosion at the navy’s testing range near the village of Nyonoksa in the northwestern Arkhangelsk region killed two people and injured six others. The state-controlled Rosatom nuclear corporation then said over the weekend that the blast also killed five of its workers and injured three others. It’s not clear what the final toll is.

The company said the victims were on a sea platform testing a rocket engine and were thrown into the sea by explosion.

Rosatom director Alexei Likhachev praised the victims as “true heroes” and “pride of our country.”

“Our further work on new weapons that we will certainly complete will be the best tribute to them,” Likhachev said during the funeral, according to Rosatom. “We will fulfill the Motherland’s orders and fully protect its security.”

Rosatom said the explosion occurred while the engineers were testing a “nuclear isotope power source” for a rocket engine. Local authorities in nearby Severodvinsk, a city of 183,000, reported a brief spike in radiation levels after the explosion, but said it didn’t pose any health hazards.

Still, the statement from Severodvinsk’s administration came just as the Defense Ministry insisted that no radiation had been released, a claim that drew comparisons to Soviet-era attempts to cover up catastrophes. Spooked residents rushed to buy iodide, which can help limit the damage from exposure to radiation.

Following the explosion, Russian authorities also closed part of Dvina Bay on the White Sea to shipping for a month, in what could be an attempt to prevent outsiders from seeing an operation to recover the missile debris. The Severodvinsk city administration said the radiation level rose to 2 microsieverts per hour for about 30 minutes on Thursday before returning to the area’s natural level of 0.1 microsieverts per hour. Emergency officials issued a warning to all workers to stay indoors and close the windows.