The best step is to query user behavior analytics (UBA) data. SIEM alerts provide potential security events, but without additional context, they may lead to false positives. UBA solutions detect anomalies by comparing user activity against baselines of normal behavior, highlighting unusual login patterns, lateral movement, or privilege escalation.

Option A (password manager logs) focuses only on credential use and lacks behavioral insight. Option B (dark web monitoring) helps identify compromised accounts but does not investigate the internal incident. Option C (audit logs for privileged actions) is useful but narrow in scope—it only covers administrator accounts.

By correlating SIEM data with UBA, the engineer can validate whether the flagged activity indicates real malicious behavior or benign anomalies. CAS-005 emphasizes advanced analytics integration (UEBA/UBA) to strengthen investigation and reduce false positives, making Option D the most effective choice.