An accidental explosion near a secretive military test site in Northern Russia has killed at least seven people, including five elite Russian nuclear scientists.

According to reports by The Moscow Times, a Russian newspaper, the explosion, which occurred on Aug. 8, was the result of a failed test of a nuclear rocket engine.

Russia state-owned nuclear research corporation Rosatom took responsibility for the explosion and explained that additional workers were killed or injured when scientists were testing a “nuclear isotope power source” on a sea-based platform owned by the Russian Navy. The explosion cast several people overboard into the sea.

The explosion, which caused a measurable release of radiation in the city of Severodvinsk 25 miles to the east, has fueled speculation that Russia may be developing a new type of nuclear weapon.

Thousands of people gathered in the nearby city of Sarov, on Monday, Aug. 12, to attend the funerals of the five Russian nuclear engineers who perished. Rosatom Director Alexei Likhachev praised each of the victims as “heroes” and the “true pride of [Russia].”

Sergei Kirienko, who is Vladimir Putin’s deputy chief of staff, also commended the victims and said that they would be awarded top medals posthumously.

President Trump made a brief comment on Twitter, on Monday afternoon, saying, “The United States is learning much from the failed missile explosion in Russia. We have similar, though more advanced, technology. The Russian ‘Skyfall’ explosion has people worried about the air around the facility, and far beyond. Not good!”