PRAGUE (AP) — Czech investigators have discovered explosives at the Palestinian Embassy complex in Prague where a possibly booby-trapped safe killed the ambassador on Jan. 1, police said Thursday.

Police discovered 12 illegal weapons following the explosion at the embassy that killed Ambassador Jamal al-Jamal, but this is the first time that authorities said explosives also were found in the new complex that includes the embassy and the ambassador’s residence.

Police provided few details about the discovery, but Deputy Palestinian Foreign Minister Taysir Jaradat said the explosives “were either part of the one that went off and killed the ambassador, or near to it but didn’t explode.”

“We rule out a political assassination,” Jaradat said in an interview with The Associated Press.

It remains unclear what caused the safe to explode, but the ambassador’s death is being investigated as a case of negligence. Al-Jamal had only started his posting in October.

Police spokesman Tomas Hulan said authorities have sent the discovered explosives for testing at Prague’s Institute of Criminology.

The Palestinians officially apologized after the Czech Foreign Ministry demanded an explanation and accused the Palestinians of breaching international obligations.

Police said Thursday they have received a letter from Palestinian authorities saying the weapons were given to them decades ago as gifts by officials in Czechoslovakia when it was a communist country. When Czechoslovakia was part of the Soviet bloc it had close ties with the Palestinian Liberation Organization.

According to police, the Palestinians said the discovered weapons were never used and always kept in a safe.

Police said experts are trying to determine whether those weapons were used in any criminal activity in the past. The ballistic testing might take weeks, police said.

Copyright 2014 The Associated Press.