def get_s3_keys(prefix, s3_client, bucket='fcp-indi'):
    """Retrieve all keys in an S3 bucket that match the prefix and site ID

    Parameters
    ----------
    prefix : string
        S3 prefix designating the S3 "directory" in which to search.
        Do not include the site ID in the prefix.

    s3_client : boto3 client object
        from the get_s3_client() function

    bucket : string
        AWS S3 bucket in which to search

    Returns
    -------
    list
        All the keys matching the prefix and site in the S3 bucket
    """
    # Avoid duplicate trailing slash in prefix
    prefix = prefix.rstrip('/')

    response = s3_client.list_objects_v2(
        Bucket=bucket,
        Prefix=prefix,
    )

    try:
        keys = [d['Key'] for d in response.get('Contents')]
    except TypeError:
        raise ValueError(
            'There are no subject files in the S3 bucket with prefix '
            '{pfix:s}'.format(pfix=prefix)
        )

    while response['IsTruncated']:
        response = s3_client.list_objects_v2(
            Bucket=bucket,
            Prefix=prefix,
            ContinuationToken=response['NextContinuationToken']
        )

        keys += [d['Key'] for d in response.get('Contents')]

    return keys