Deep in the heart of Green River Killer country our initial goal was to try to identify probable serial killer victims. We did that and found a lot more, but not without a struggle. We learned that when someone went missing his or her records often disappeared too. That was one of the challenges reporters at the Post-Intelligencer faced when we attempted to analyze 20 years of missing-persons reports from more than 300 police agencies in the state of Washington.

The police had misplaced or purged dozens of reports for people still listed as missing in the official state and national databases. The official databases also were inaccurate. Many people who should have been included weren't, and other cases that should have been purged remained in the system. The consequences were dramatic: Murder cases went unsolved, bodies remained unidentified and killers stayed free.

The databases we built and those we obtained from police, coupled with traditional investigative reporting, allowed us to make such sweeping conclusions with confidence. We also found that agencies had lost or destroyed more than 100 reports for active missing-persons cases. Coroners and medical examiners had lost files, physical evidence and even remains. Most police agencies routinely violated a law that requires them to seek dental records in long-term missing person cases, even though the records can provide key clues in identifying corpses.

The database steered us to dramatic human stories: a foster child whose disappearance and mysterious death in Mexico was never adequately investigated. A toddler, the unidentified victim of murder in 1987, whose image had never been publicized until our series ran. A missing woman whose frantic family could not convince police to take a report after she slit her wrists and drove away. The anecdotes alone were appalling but our database allowed us to combine the tales with hard numbers.

Our investigation took nearly a year, mainly because it was so difficult to obtain records, most of them police reports from open investigations. But it also took time to track down families of the victims. Reporter Lewis Kamb filed the first public disclosure requests in January 2002, asking all the departments across the state for unsolved missing persons reports. From the beginning, the newspaper specified that it wanted reports that were at least a year old.

We didn't want to track the thousands of teen-agers and adults each year who return home after a few days. Federal and state auditors regularly are supposed to review whether police accurately track long-term missing persons cases in the National Crime Information Center (NCIC) database. But it immediately became clear that most departments didn't have their records in order. While some departments responded to the public records requests within a week or two, most dragged their feet.

Some flatly refused to provide records. Others provided information only in summary form or redacted names. It took the Seattle Police Department, the state's largest agency, nine months to respond. Kamb was relentless in his pursuit of the records. He pointed out to the reluctant police departments that some major police agencies had immediately provided copies. After several months, I got involved in building the database.

Kamb had boxes full of reports. But, using a list of official missing persons provided by the state, I quickly realized that many departments had not given us everything. We wrote back again and called some departments dozens of times. By late fall, we had assembled more than 600 reports in all sorts of formats, some handwritten, some barely readable, but most usable. I built an index to our reports that included fundamental information, such as the department name, date, victim name, age, sex, race, where disappeared, usual residence, a description of the victim and a summary of circumstances behind the disappearance.

I marked others that seemed to bear more investigation as "interesting" in the database. I also attempted to categorize the reports by the reasons police had given in the reports to possibly explain the disappearance. Foul play was suspected in many. A key category for us was serial killer victim. This involved a lot of investigation into alleged serial killers who had operated in Washington since the 1980s.

In additional to the missing-persons database, I built a database of unidentified dead persons. Next we compared the information more closely to official records. We received parts of the missing persons data and much of the unidentified persons data from the state. Journalists can obtain similar data from the authorities in their own states. We found that more than 60 percent of all missing persons had no dental reports on file.

Despite the limits of the data, I was able to analyze it and discover that some probable murder victims had never been entered as official missing-persons. The NCIC tries to match the tables of the missing and the dead but, our reporting found, the automated search engine in the database was flawed. So I matched the tables we obtained to see whether I could find matches overlooked by the police. So far, the identities of two long-unidentified bodies have been confirmed based on information generated by the series.

I hope that more will be added to that list over time. To see the series and a portion of the databases we used, see http://seattlepi.nwsource.com/missing. Contact Lise Olsen by e-mail at LiseOlsen@seattlepi.com.