No one is really sure, which is a pretty unsatisfactory answer. The original radiation measurements were performed with a gamma spectrometer mounted on a helicopter, which is not useful for finding radioactive contamination in the ground - and the officials who knew this at the time, yet claimed the site was clean, are now in a lot of trouble.

https://sf.curbed.com/2020/1/23/21075707/shipyard-radiation-...

Side note, Treasure Island also has, or had, a pretty serious radioactivity contamination. I've measured slightly elevated emissions there myself, as a matter of fact.