Palestinians launched an investigation Monday after seven people were hospitalized when one of them opened a suspicious envelope addressed to Palestinian Prime Minister Ismail Haniyeh, officials said. Haniyeh, a leader of the militant Hamas group who heads the Palestinian government, said he believed this may have been an Israeli attempt to kill him with "poison gas." "We don't rule out the involvement of the Israeli security services, which indicates a dangerous mentality," he said in a speech to the Palestinian Parliament in Gaza. The Palestinian Cabinet building in the West Bank city of Ramallah, where the envelope was delivered, was evacuated, said Deputy Prime Minister Nasser Shaer. The envelope contained an orange tissue that emitted a strong smell, said Shaer's office manager Abdel Basit Moatian, who opened the mail. Moatian said it had a Tel Aviv postmark. Israel has tried to poison Palestinian leaders in the past and there were widespread rumors that Israel poisoned the late-Palestinian leader Yasser Arafat, an accusation Israel vehemently denied. Israeli officials declined to comment on the report. A security guard who handled the envelope and another woman who was in the room at the time were hospitalized and given oxygen, local hospital director Hosni Atari said. Atari said they were complaining of strong headaches and had fainted, but were now conscious. Five other people who came in contact with the envelope, including Moatian, were briefly admitted for checks, he said. "It was a bad, bad strong smell," said Ghader Ismail, the woman who was hospitalized. Palestinian officials said it was not clear what the substance was or if it may have been an attempt to a member of the Hamas government. After Hamas won January parliamentary elections, Israel boycotted the Palestinian Authority, refusing to deal with a group it considers a terror organization. In June, Hamas-linked militants from Gaza attacked and Israeli army post and captured a soldier, a raid that sparked a wide-scale Israeli offensive in Gaza. "I can't rule out that someone was targeting someone in government, but I can't confirm this," Deputy Health Minister Anan Al Masri said. "I can confirm that there was a strange substance," he said, adding that tests were being carried out to determine what it was. Moatian said workers in the building became suspicious of the envelope because it was mailed to Haniyeh in the West Bank, even though Haniyeh lives in Gaza and is banned from traveling to the West Bank by Israel.