Sourcing -- identifying, evaluating, and using information about the sources of information -- assists readers in determining what to trust when seeking information on the Internet. To survive in the post-truth era, students should be equipped with sufficient sourcing skills. This study investigated the efficacy of a teacher-led intervention aimed at fostering upper secondary school students' (N = 365) sourcing during online inquiry. The intervention (4 × 75 min) was structured in accordance with the phases of online inquiry: locating, evaluating, synthesizing, and communicating information. During the intervention, teachers demonstrated why and how to source, and students practiced sourcing by investigating a controversial topic on the Internet. Students worked in small groups and their work was supported with analysis and reflection prompts. Students' sourcing skills were measured with a web-based online inquiry task before and after the intervention. Compared to controls, the intervention fostered students' abilities in three of the four skills measured (sourcing in search queries, credibility judgments, and written product). Depending on the sourcing skill, 4-25% of students showed improved performance. The students with low sourcing skills to begin with, benefited the most from the intervention. The study demonstrated that students' sourcing skills can be supported throughout online inquiry.