Complex sentence: Unknown to the crime family, Eugene is a reluctant informant for the FBI, which has increased its pressure on Pontecorvo to produce information, having lost an asset when informant Raymond Curto died suddenly of a stroke while talking to Agent Robyn Sanseverino.
Split into simpler sentences: Unknown to the crime family, Eugene is a reluctant cooperator with the FBI. The Bureau have stepped up their pressure on Pontecorvo to produce information, having lost an asset when informant Raymond Curto died suddenly of a stroke while talking to Agent Robyn Sanseverino.