I'll post a bit of background in a comment for those not in the loop.
Glenn Greenwald's (https://en.wikipedia.org/wiki/Glenn_Greenwald) The Intercept has published leaked conversations (from 2015-2018) between prosecutors and Sergio Moro, the judge in Lula's case. The leaks reportedly comprise "over 1700 pages" of Telegram conversations.
In the small portion of the leaks published so far, Moro suggests how operation Carwash should proceed, indicates possible witnesses, directs and counsels the prosecution on how to present evidence and even anticipates his rulings. Lula's defense points out this is grounds for annulment of the entire case. Brazil is currently split in polarization and the leaks are taking precedence over other political matters which until last week dominated the discourse (such as the proposed welfare reforms).
Moro and the Prosecutor's Office did not deny the conversations took place but Moro did not explicitly confirms the contents are true. The Intercept and other journalists are being attacked about the 'illegal' nature of the leaks, mostly, which makes them unusable for criminal prosecution of Moro and prosecutors but can still be used politically and/or might result in overturning many Carwash sentences - including Lula's.
About Telegram and the leaks:
Telegram states "there's no evidence of any hack. Most likely to have been either malware or someone not using a 2-step verification password."
Today, however, someone hacked into a Brazil's Public Prosecutor's Office Telegram group - at first impersonating a member. They later revelead that they were an impostor and stated that they were the hacker responsible for the leaks. If this is correct, the hacker (who states to not be politically motivated and "just an IT guy") can still (reliably?) gain access to Telegram's conversations and is also capable of impersonating people.
Lula (left-wing politician) was Brazil's president for two terms (2003 through 2010) and was followed by Dilma Roussef trailing off of his extremely high popularity. Dilma was impeached in 2016, midway through her second term, following the political scandals brought to light by operation 'Carwash' (Lava Jato), which comprises until now over a thousand warrants for search, arrests, and so forth. Her impeachment was highly debated and to this day Lula's party (PT, "worker's party") claims it was a "bland coup".
Judge Sergio Moro, who judged most of the cases involving high corruption, achieved very high popularity and opinions about him divided the country. Through several extremely controversial actions he influenced Dilma's impeachment, and later the 2018 election. Since then Moro is accused of being a partial, politically inclined judge, persecuting unfairly several high-ranking members of PT. He has always denied that.
Regardless, the public opinion has kept Operation Carwash in very high regards with a sizeable portion of the population and is credited with helping elect Jair Bolsonaro, Brazil's current president, by fostering the 'anti-PT' movement. Lula was sent to jail in a _extremely_ controversial ruling by Moro and several other legal manouvers (leaks, stopping him from giving interviews, which is unconstitucional, for example). NEvertheless, none of the decisions have been overturned and Brazil's Supreme Court generally allowed all these controversial actions.
Surprisingly, Lula's popularity didn't completly waver and his appointed candidate (Haddad) made it to the second round of the election, although he lost to Jair Bolsonaro.
Accusations against Moro mounted after he accepted an invitation to become Bolsonaro's minister of Justice, benefitting from the election of the man he helped elect, and after Bolsonaro stated he would indicate him to the next Supreme Court seat.