The extradition request arrived in 2012, but they managed to keep it secret until this arrest. Now that official press releases have come out from both countries, we know the charge underlying the extradition. The fact sheet linked from the UK press release explains that the rules for UK->US extradition would require consent from the UK, including consideration by the UK courts, of any additional offenses that the US may want to add while they have him in connection with this extradition.

https://homeofficemedia.blog.gov.uk/2019/04/11/extradition-f...

They may very well approve some additional offenses, but probably not ones with the death penalty.