Official lab tests of a suspicious letter sent to the office of Palestinian Prime Minister Ismail Haniyeh detected no known poisons, a senior Palestinian official said Tuesday. The letter had a Tel Aviv postmark and arrived Monday in Haniyeh's office in the Cabinet building in the West Bank town of Ramallah, according to Palestinian officials. Two employees in the Cabinet were overcome by a strong smell after opening the letter and were later hospitalized, complaining of headaches, breathing difficulties and weakness in the legs. In all, some 20 employees reported to the Ramallah hospital, some as a precaution, Palestinian officials said. The envelope was sent to the West Bank's Bir Zeit University for testing. A senior Palestinian government official, who spoke on condition of anonymity, said the lab did not detect any known poison, but that tests would continue.