It has been 10 years since the costliest ecoterrorism arson in the nation ripped through the construction site of a University City housing project, a $50 million inferno that was unforgettable to anyone awakened by the towering flames.

The five-story apartment complex has long since been rebuilt, but the FBI file on the case is far from closed.

A banner left behind at the scene of the 2003 fire, as well as four others placed at another series of arsons weeks later, left no doubt it was the work of the Earth Liberation Front, an international radical environmentalist group that aimed to fight urban sprawl through economic sabotage. But identifying exactly who set the fires remains a mystery that the FBI has spent the past decade trying to solve.

These days, much of that investigation centers around the same network of activists, as agents interview and reinterview them year after year hoping something within has changed — an ideology, a relationship, a moral tug.

“If somebody truly believes and buys into that (militant) mind-set, it’s going to be difficult for us to get any kind of cooperation,” said FBI Supervisory Special Agent Keith Kelly.

“But over time, that can erode, when folks have different perspectives and different life experiences.” Kelly said. “People realize the actual environmental impact to extinguish a fire is far more damaging. Those types of things start to creep in, and it starts to help us break down those barriers.”

And then there’s the little mystery within the mystery, the color-coded clues on the backs of the banners that no one has been able to figure out.

The fires

An apartment complex at 9805 Judicial Drive was the site of an Earth Liberation Front arson 10 years ago while it was in the framing process. — John Gastaldo

The sprawling La Jolla Crossroads residential and research complex currently sits on 33 acres between the University Towne Centre mall and Interstate 805. It’s popular with University of California San Diego students and young professionals, many who have no knowledge of the complex’s explosive beginnings.

The project’s developer, Stuart Posnock, was asleep on Aug. 1, 2003, when he got a 3 a.m. call from a friend alerting him to the fire. As he drove down Mount Soledad, the predawn sky glowed ominously orange above his construction site.

He pulled up just as a 100-foot crane toppled to the ground. Flames soared 200 feet into the air, incinerating the framed 206-unit apartment complex. Hundreds of nearby residents had evacuated. Fireballs landed on neighboring patios.

No one was injured, but the fire had done its damage.

On the ground were two white bedsheets laid together, their corners weighted with heavy rocks. “If you build it — we will burn it,” the banner read. “The E.L.F.s are mad.”

The arson put other builders around the county on alert as they added security cameras and overnight guards to further protect their sites.

ELF struck again in the early morning hours of Sept. 19, 2003, at construction sites in Torrey Highlands. Four unfinished homes were destroyed and two were damaged at Shea Homes’ Avalon Point and Pardee’s Bordeaux tract. The arsonists tried to burn a third site, Western Pacific Housing’s Monaco development, but the blaze never really took off.

The total damage was put at $3 million.

Again, ELF broadcast its message with calling-card banners: “Environmental murder. Nature demands justice. The E.L.F.s are mad.”

The investigation

This FBI photo shows a banner found at the University City site. It's 12-feet long and made of two bedsheets put together. — Photo courtesy of the FBI

ELF officially claimed responsibility for the blazes on its website and in an email to The San Diego Union-Tribune.

Because ELF is considered a domestic terror organization, the FBI in San Diego took the lead on the case. At least 19 other agencies assisted, including San Diego arson investigators and a special response team with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives .

The five banners collected at the scenes held an especially puzzling clue, one that investigators haven’t revealed publicly until now.

The backs of each set of banners were labeled as a different shade of purple. The two sheets at the University City blaze, for example, each had “magenta” written on the back. Each pair of sheets at the other sites got their own labels: mauve, violet, lavender and burgundy.

“The meaning of that is still unknown. Only the people that wrote it would be aware of it,” said FBI Special Agent Justin Menolascino, the lead case agent. “Is it code to keep the different pieces of the banner together? That begs the question: Why not just use a numbering system?”

Investigators said that besides the banners, the fires are tied together by the time of day, method of ignition and location of ignition.

FBI Supervisor Kelly described the ignition method as somewhere between “crude” and “sophisticated,” a home-built device made with readily available materials. He declined to specifically describe the devices for investigative purposes.

“It’s not a gas can with a rag in it. There is a mechanism and a system for this thing to ignite,” Kelly said. “It is somebody who really knows the mechanics of how this has to work the right way. ... It’s not that difficult if you know what you’re doing.”

Information on building such devices was widely circulated among extremist networks. One manual published online in 2001 called itself an arson guide for ELFs, explaining how to use electronic timers, where to put incendiary devices and how to avoid leaving behind DNA.

And longtime activist Rodney Coronado, who referred to himself as an ELF spokesman, demonstrated to a crowd how to build an arson device during a speech in Hillcrest hours after the enormous University City blaze.

The suspects

Coronado’s speech was a logical place for the investigation to start. He was not considered an arson suspect, because he was home in Tucson, Ariz., when the fire was set.

Agents focused on who else attended the speech, which was organized by the local chapter of Compassion for Farm Animals. The FBI served several search warrants at the homes of animal-rights activists who attended, and three of them were later held in contempt of court when they refused to testify before a federal grand jury in 2005.

Federal authorities never called them suspects but said they believed the trio had information pertinent to the investigation. In news interviews, the three said they weren’t involved with ELF.

Attempts to reach them for this story were unsuccessful.

Coronado was ultimately held liable for that speech. He was sent to prison for a year after pleading guilty to demonstrating how to make a destructive device.

But the fires have remained unanswered for.

Part of the difficulty lies in the loose construction of the group, with no central leadership and cells that are encouraged to act autonomously.

The case has passed through several hands over the years, landing in Special Agent Menolascino’s most recently.

The FBI estimates hundreds of people have been interviewed in the past decade.

Part of Menolascino’s job is to review all of the past agents’ work to look for holes or opportunities missed. Then he goes back at those same people all over again in hopes that another interview will reveal something new. Maybe a fresh eyewitness detail. Or maybe an activist with a change of heart.

“It’s amazing how people do change their perspective,” said Kelly, who supervises Menolascino on the FBI’s domestic terrorism squad.

It seems the ELF movement as a whole has shifted its views away from arson as an acceptable protest method.

A web page claiming to be the unofficial but legitimate ELF site urges activists to channel their energies into building public consensus and support rather than destroying property.

“Even if the attack does not injure or kill an innocent person, there are no winners. Torching sport utility vehicles, ski resorts, research labs and McMansions releases huge amounts of toxic gasses into the atmosphere — creating far more greenhouse gasses than if they were left alone,” it says. “The end result: everything is rebuilt, replaced or repaired. This DOUBLES the burden on the environment and taxpayers! An exercise in futility and self-defeat.”

ELF arsons have become rare these days compared to the late 1990s and early 2000s, when the group claimed responsibility for torching a Vail, Colo., ski resort, an agricultural hall at Michigan State University , luxury homes in Washington state and tree farms in Oregon.

The FBI has had some success in making arrests in those cases, including several linked to a Eugene, Ore., cell called The Family.

Meanwhile, in San Diego, ELF’s sister group, the Animal Liberation Front, has been busy sending its own destructive animal-rights message. The group claimed responsibility for vandalizing a Clairemont fur store, Furs by Graf, as well as the East County homes of its two owners, in July.

The store was spray-painted, its windows etched, its locks glued shut and its interior doused with acid. The same FBI squad is investigating.

Menolascino said that while some of his fellow agents register surprise when they learn the decade-old arson case is still open, he promised that the investigation isn’t in vain.

“We are definitely further along than where we were in 2003,” Menolascino said. “You look at 10 years of investigators putting all the pieces of the puzzle together. Things keep coming together slowly. We’re learning more and more all the time.”