Traces of explosives have been found on the remains of victims of the EgyptAir flight that crashed in the Mediterranean Sea in May, the country’s civil aviation ministry said on Thursday. The ministry said it would initiate a criminal investigation into the case, reported The Guardian.

“The central directorate of aircraft accident investigation received reports from the forensic medicine authority indicating traces of explosive materials found in some of the remains of the victims’ bodies,” the ministry said. However, investigators have yet to reveal how explosives could have been smuggled into the flight. All 66 people on board were killed in the incident. The flight took off from Paris and was on its way to Cairo.

In July, Egyptian investigators said the word “fire” was heard on the plane’s cockpit voice recorder before it crashed. The black box also confirmed that smoke alarms had gone off before the crash. French investigators had reportedly said in September that they had found traces of the explosive TNT on the debris. However, they had claimed that they were not allowed to investigate further.

Egypt’s aviation minister had also held that the chances of a terrorist attack were “higher than the possibility of a technical [failure]” in the case. Families of five French victims have launched a petition to recover the remains of those killed in the crash.

The crash on May 19 took place about seven months after a Russian passenger plane was brought down by a bomb over Egypt’s Sinai peninsula. The Islamic State group claimed responsibility for the attack that killed all 224 people on board.