Participants enrolled in the study after Amendment 3 were randomized in a 1:1:1 ratio to receive adalimumab (1 of 2 regimens) or placebo during the 8-week DB induction period. Participants received DB therapy from Baseline until Week 8 and OL therapy from Week 8 until the end of the study. In the first adalimumab dosing arm (adalimumab 80/40), participants received 2 injections of adalimumab 40 mg (80 mg) and 2 injections of placebo at Baseline followed by 1 injection of adalimumab 40 mg and 1 injection of placebo at Week 2, and 1 injection of adalimumab 40 mg every other week thereafter. In the second adalimumab DB induction dosing arm (adalimumab 160/80), participants received 4 injections of adalimumab 40 mg (160 mg) at Baseline followed by 2 injections of adalimumab 40 mg (80 mg) at Week 2, and 1 injection of adalimumab 40 mg every other week thereafter. Participants randomized to placebo received 4 injections of placebo at Baseline followed by 2 injections of placebo at Week 2, and 1 injection of placebo at Weeks 4 and 6. Beginning at Week 8, but after the Week 8 study assessments had been completed, all participants received 1 injection of OL adalimumab 40 mg every other week until Week 52 or early termination. Starting at Week 12, participants who had inadequate responses to treatment (as defined using partial Mayo scores) were permitted to dose escalate to adalimumab 40 mg weekly. Participants with persistent inadequate response while on weekly treatment could be discontinued from the study at the discretion of the investigator.