On postoperative day 2, after elective sigmoid resection and primary anastomosis for diverticular disease, a patient reports multiple bright red bloody stools that persists over the next 12–24 hours. Initially, he had a heart rate of 110 and blood pressure of 100/50 mmHg, and he sustained a hemoglobin drop from 12 g/dL to 8 g/dL, requiring transfusion. After resuscitation, what is the best next step in management?