A federal investigation expanded Thursday as prosecutors subpoenaed a fourth peace activist to appear before a grand jury and secured a court order forbidding Drake University officials to discuss a demand for information about a November antiwar conference on campus.

Federal authorities so far have refused to disclose what the investigation is about or what laws might have been broken. But an officer with the FBI's Joint Terrorism Task Force served a subpoena to Wendy Vasquez of Des Moines on Thursday morning.