The head of the Russian nuclear agency said Monday that the five scientists killed last week in a rocket explosion at a missile test range had been developing “new weapons,” according to a report.

The accident occurred Thursday during tests on a liquid-propellant rocket engine at an arctic naval range on the coast of the White Sea in Nyonska run by state nuclear company Rosatom.

US experts have said the blast could have been related to testing of the Burevestnik cruise missile, known by NATO as SSC-X-9 Skyfall, which President Vladimir Putin touted earlier this year.

As a memorial service was held for the scientists Monday, Rosatom chief Alexei Likhachev said their efforts would continue, according to Agence France-Presse.

“The best (thing) for their memory will be our further work on the new weapons,” he said. “We are fulfilling the task of the motherland, its security will be reliably ensured.”

Sergei Kiriyenko, deputy head of Putin’s administration and a former nuclear chief, called the victims “real heroes.”

He said the scientists took on physical risks “which, unfortunately, however much you prepare, cannot be completely avoided.”

On Saturday, Rosatom acknowledged that its staff was providing engineering and technical support for the “isotope power source” of a missile.

The Russian military announced the death of two “specialists” after the explosion but it was not immediately clear if they were among the five scientists whose deaths were announced by Rosatom.

Three others were injured in the accident, suffering burns, according to the nuclear agency.

At first, the military did not say the accident involved nuclear equipment, stressing that radiation levels were normal afterward.

But the nearby city of Severodvinsk recorded elevated levels after the accident and panicked residents rushed to buy iodine to counteract radiation.

In an address to the nation earlier this year, Putin announced the development of what he called “invincible” missiles, threatening to deploy them against “decision-making centers” in the West if there were serious threats against Russia.