By Kanishka Singh WASHINGTON (Reuters) -U.S. President Joe Biden said on Friday he was “heartbroken” by the news that an American named Gadi Haggai is believed to have been killed by Palestinian Islamist group Hamas on Oct. 7 when it attacked Israel. Haggai, a 73-year-old Israeli-American man, was previously thought to have been taken hostage in the attack, along with his wife. A group representing hostages’ families had said earlier on Friday that Haggai died in captivity. “Jill (Biden) and I are heartbroken by the news that American Gadi Haggai is now believed to have been killed by Hamas on…