def filter_common_word(possibles: list[str], common_word: str) -> list[str]:
    """
    Given a list of possible decoded messages, narrow down the possibilities
    for checking for the presence of a specified common word. Only decoded messages
    containing common_word will be returned.
    >>> filter_common_word(['asfla adf', 'I am here', '   !?! #a'], 'am')
    ['I am here']
    >>> filter_common_word(['athla amf', 'I am here', '   !?! #a'], 'am')
    ['athla amf', 'I am here']
    """
    return [possible for possible in possibles if common_word in possible.lower()]