（what） I told you just now was （what） had been written in th

（what） I told you just now was （what） had been written in the letter.
第一个what是因为句子中缺少主语成分,我知道；不明白第二个what.是因为be动词后的宾语从句一定要用what吗?请从专业角度解释为什么不用that啊.