3GPP TS 33.107 describes about the Lawful Interception & 33.108 specifies about the HI interfaces
However, the specifications do not define the the X1_1 message flows (message contents) between the ADMF & the Network Entity. Any thoughts?
3GPP Evolved Packet System (EPS) provides IP based services. Hence, EPS is responsible only for IP layer interception of Content of Communication (CC) data. In addition to CC data, the Lawful Interception (LI) solution for EPS offers generation of Intercept Related Information (IRI) records from respective control plane (signalling) messages as well.
For more details Lawful Interception Architecture for LTE Evolved Packet System
How Lawful Interception works in LTE EPC and how it is different then UMTS...