The only suspect ever charged, Minneapolis activist and graduate student Scott Ryan DeMuth, has always maintained his innocence. Under a plea bargain in September, federal prosecutors dropped allegations that he played a role in the attack, and he instead pleaded guilty to a misdemeanor for helping in a 2006 break-in at a ferret farm in Minnesota.

Barring a surprise development, that means nobody will be held criminally responsible for the raid in which masked men broke into Spence Laboratories, freed 400 rats and mice, dumped chemicals on data, damaged computers and equipment and publicized researchers' home addresses. The university estimated the damage at $500,000, not including the years of research lost.

"While active investigation of the attack on the University of Iowa Laboratory has been completed, there are still ongoing investigations into other potential criminal activities that came to light during the course of the investigation," Weysan Dun, special agent in charge of the FBI's Omaha office, said in a statement Monday.

DeMuth's lawyer, Michael Deutsch of Chicago, said the five-year statute of limitations on the attack has run out and the FBI "hasn't come up with anybody." The government could still theoretically bring charges against suspects by arguing a criminal conspiracy existed for years afterward, he said.

UI psychology researchers whose work was destroyed still call the Nov. 14, 2004, attack an act of domestic terrorism. They said they are disappointed nobody will be held responsible, but praised the FBI's aggressive pursuit of the case.

"While this wasn't the outcome we had hoped for ... the message was clearly sent that felony acts committed in the name of liberating animals are not going to be tolerated," UI Psychology Department Chairman Alan Christensen said.

But Deutsch said the investigation into DeMuth, who was originally indicted on one count of conspiring to commit animal enterprise terrorism, was an overreach.

DeMuth came under scrutiny after authorities investigating alleged criminal acts by protesters during the 2008 Republican National Convention in St. Paul, Minn., raided his home and seized his computer and a journal. In a 2005 entry, he wrote he was worried about federal scrutiny and that "it's been almost a year since Iowa" but did not elaborate.

Authorities said DeMuth's height was similar to one of the masked intruders seen in a video released by the Animal Liberation Front, an extremist group that took responsibility for the attack.

Deutsch said the journal entry was referring to a 2004 meeting of protesters in Des Moines, not the university attack, and he noted the identities of those in the video could not be ascertained because of their disguises.

DeMuth contends his indictment was vindictive, coming after he refused to answer questions before a grand jury, and that he was targeted because his political views are anarchist. The charge also came days before the statute of limitations ran out.

His former girlfriend, Carrie Feldman, was detained in connection with the investigation but released without being charged.

"They made a desperate effort to haul Mr. DeMuth into it, but he wasn't involved in it in any way," Deutsch said. "It just seemed like they were desperate to hold somebody accountable. The statute was about to run out, and they falsely accused this young man of being involved. The FBI has put in a lot of resources to figure it out and hasn't come up with anybody."

But UI psychology professor Amy Poremba, whose research was among that destroyed, scoffed at the notion that DeMuth was innocent. The 23-year-old faces prison time when he is sentenced in January for the April 2006 release of ferrets from Lakeside Ferrets Inc., a ferret breeder in Minnesota.

"By pleading guilty to the ferret break-in, Demuth admits he is clearly connected to groups willing to break the law," Poremba said. "The FBI investigation was very useful in gaining information about the population involved in these terror activities."

Dun said the case was a priority for the FBI, which collaborated with federal prosecutors, UI police and the Joint Terrorism Task Force. Hundreds of interviews were conducted, and investigators collected and analyzed extensive amounts of evidence, he said.

An FBI affidavit last year spelled out the painstaking investigation, which concluded the overalls worn by suspects were bought in Cedar Rapids, and the Animal Liberation Front may have sent its message taking responsibility from a computer in UI's law library.

Investigators raided the home of an activist in Salt Lake City in connection with the case, but that person, Peter Daniel Young, was never charged.

Young, who has been convicted in a string of incidents in which he freed mink from fur farms, called the case "among the most egregious examples of prosecutorial overzealousness in the animal liberation movement's history" on his blog. He denied involvement in the Iowa attack, which he called one of the largest and most successful on a university research lab.

"They were able to get in deep inside a laboratory that had some fairly sophisticated security," he said. "They were able to get animals out, smash the labs up and not be apprehended. That was very empowering for lots of activists."