public static function mark_as_read( $thread_id = 0 ) {
		global $wpdb;

		$user_id =
			bp_displayed_user_id() ?
			bp_displayed_user_id() :
			bp_loggedin_user_id();

		$bp     = buddypress();
		$retval = $wpdb->query( $wpdb->prepare( "UPDATE {$bp->messages->table_name_recipients} SET unread_count = 0 WHERE user_id = %d AND thread_id = %d", $user_id, $thread_id ) );

		wp_cache_delete( 'thread_recipients_' . $thread_id, 'bp_messages' );
		wp_cache_delete( $user_id, 'bp_messages_unread_count' );

		/**
		 * Fires when messages thread was marked as read.
		 *
		 * @since 2.8.0
		 *
		 * @param int $thread_id The message thread ID.
		 */
		do_action( 'messages_thread_mark_as_read', $thread_id );

		return $retval;
	}