The US attorney's office had no immediate comment on why the subpoenas were withdrawn.
On Monday, prosecutor Stephen O'Meara had issued a statement that its investigation was focused on alleged trespassing on Nov. 16 at the Iowa National Guard headquarters in Johnston, which happened while a protest against the war in Iraq was taking place nearby. He said the protest, in which 12 were arrested, was not the problem.
The antiwar forum at Drake University was held the previous day.
Al Overbaugh, a spokesman for the US attorney's office, said the investigation was not over, but he would not comment further.
As part of the probe, prosecutors had served a subpoena last week asking the university to turn over the names of participants in the forum. It also requested campus security records about the forum, sponsored by the Drake chapter of the National Lawyers Guild. The forum included sessions on nonviolence training and the Iraq war.
Drake was preparing legal motions to fight the subpoena when Steve Serck, a lawyer representing the school, received news it had been dropped.
"We would have argued that it chilled the First Amendment rights of free speech and free association of our students," he said.
Civil liberties advocates welcomed the withdrawals but said troubling questions remain. The Iowa Civil Liberties Union pledged to file legal motions and "use other avenues" to find out why the subpoenas were served in the first place.
"If it was just a trespassing investigation, why seek the membership records of the National Lawyers Guild?" asked Ben Stone, executive director of the ICLU.