Japan's ambassador to the US delivers apology for his country's bombing of the USS Panay in the Yangtze River

The transcript details an incident where the American gunboat Penne was bombed and sunk by Japanese naval airplanes in the Yangtze River. The Japanese government acknowledges the mistake and expresses sincere regret to the American government and people for this unfortunate event.

What was described as a great mistake in the text?

What is the tone of the message from the Japanese government?