Security has been increased at Brussels International Airport after a letter threatening attacks against the Israeli airline El Al was found in a toilet room over the weekend, a police spokeswoman said Monday. "I can confirm we found a letter in a toilet room of the Zaventem airport on Sunday," said federal police spokeswoman Evie Claes. "The letter is being analyzed. While we wait for the outcome of the analysis, we have added extra police patrols at the airport," she said, giving no details of the letter. A spokeswoman for the Brussels prosecutor's office said the threat level was being evaluated. "We cannot disclose the content of the letter. An investigation is under way," said Estelle Arpigny. The El Al office in Belgium said it had been informed of the letter but had no comment to make. An official at the El Al office in Brussels, speaking on condition of anonymity, said the airline is well-guarded and continues operations as usual. The scheduled El Al flight from Brussels to Tel Aviv on Monday morning was not affected by the threat and left on time, said airport spokesman Paul De Backer. Some 5,000 people marched through Brussels on Sunday to protest the IDF offensive in Lebanon and to demand an immediate cease-fire, in the first large-scale protest in the Belgian capital since the fighting broke along the border between Israel and Lebanon on July 12. Belgian news media have been critical of the IDF offensive.