Twitter permanently deletes tweets deemed crucial in Russian investigation

Twitter has deleted tweets which were deemed important for investigators in the ongoing examination of Russia's suspected manipulation of the social network during the 2016 presidential election according to the US officials.
Twitter's policy means that once a user deletes a tweet, the tweets will be permanently deleted from Twitter's servers after 30 days through an automated process.
As a result, any of the account holders who deleted tweets and the accounts suspected of having spread false or