[Police International Monetary Fund (IMF) Offices Paris France]REUTERS/Philippe Wojazer A letter exploded when it was opened at the offices of the International Monetary Fund in Paris on Thursday and slightly injured an executive assistant's face and hands, a police source said. According to the first elements of the investigation, a firecracker could be at the origin of the explosion, which occurred late Thursday morning, according to the French newspaper Le Figaro. Several people were evacuated "as a precautionary measure," according to Le Figaro, and the Parisian judicial police opened an investigation. The explosion was caused by a homemade device, according to the head of the French capital's police force on Thursday. "It was something that was fairly homemade," the Paris police chief, Michel Cadot, told reporters. Cadot also said some telephone threats had been made in recent days but it was not clear whether these were linked to the incident at the IMF's offices. A police intervention is underway near the World Bank, which is located in the same offices as the IMF. "An envelope exploded after it was opened, and one person was slightly injured in the offices of the IMF," one police source told Reuters. The incident, just six weeks before a presidential election, comes as a militant Greek group, Conspiracy of Fire Cells, claimed responsibility for a parcel bomb mailed to German Finance Minister Wolfgang Schaeuble on Wednesday. The head of the IMF, Christine Lagarde, said: "I condemn this cowardly act of violence and reaffirm the IMF's resolve to continue our work in line with our mandate. We are working closely with the French authorities to investigate this incident and ensure the safety of our staff." President Francois Hollande said French authorities would do all they could to find those responsible for the incident. NOW WATCH: A Navy SEAL explains what to do if you're attacked by a dog