The Federal Communication Commission's online political ad database is supposed to make information about heavy political hitters more accessible, but a lack of clarity in the rules has resulted in some stations effectively censoring what the public is permitted to see.

An analysis using Sunlight's Political Ad Sleuth, a project to organize and expand the FCC database, shows that of the more than 220 stations that are required to post their political files online, more than half have removed documents since the process began Aug. 2. More than 2,100 of the total 35,400 records appear to have been taken down. Of the apparently missing files, ProPublica has 1,608 on Document Cloud, an online service that journalists use to store information for stories. Because of the disturbing pattern of disappearing documents, Sunlight has begun backing up all FCC filings. The FCC did not respond to numerous calls requesting clarification on the rules.

Differences in how broadcasters are maintaining their online files, however, suggested that at least some of them are doing something wrong.

MORE: Help unlock the political ad files.

Before Aug. 2, TV stations were required to keep political ad information in paper form. After a lengthy court battle with the National Association of Broadcasters, the Federal Communications Commisson began requiring TV stations affiliated with the top four networks — ABC, CBS, NBC, and Fox — in the top 50 markets to upload their political files to a database.

With paper files, it was common practice to include original ad agreements along with any revisions; online, many stations are constantly deleting old agreements and replacing them with newer versions, keeping the public in the dark on the changes. Updates to ad files can show that, while the same amount may be spent, the commercial times change, perhaps to target a different demographic. In other cases, they may show increased or decreased spending, demonstrating a significant shift in strategy from the campaign or spending group.

Other stations choose to upload a revised order form, allowing for easier tracking of the numbers.

How political files are processed depends on an interpretation of FCC guidelines, handled by the legal team at each station. As one sales manager, who preferred not to be identified, said of the FCC rules: "You'll go to 50 different stations and get 43 different interpretations."

Broadcasters disagree on what advertising documents need to be included. Some only upload agreements that have "national significance," leaving out state and local candidates, as well as spots dealing with issues that don't extend beyond state lines. Others maintain robust state and local files.

Because of the freedom allowed, mistakes often arise in uploading the files to the FCC database. Labeling is inconsistent across the board, sometimes even within the same station. Some broadcasters include the same document twice under a different name, or upload incomplete NAB forms. These forms are supposed to include the names of principals of the committees buying the ads and can provide critical information to identify the agendas of some of the vaguely-named groups that have been pumping millions into campaigns.

At Las Vegas CBS affiliate KLAS, just the folder related to Danny Tarkanian, Republican nominee for Nevada's 4th district seat, turns up a number of discrepancies. Of the three ad agreements listed on the FCC site, two sets of documents have been deleted and revised, and the revised order forms aren't clearly labeled. One order for spots running between Oct. 8 and Oct. 16 was deleted twice; while the NAB agreement form for the ads remains online, the order initially wasn't there, and was replaced only after a reporter pointed the absence out to station officials.

An employee there insisted the omissions and errors are not malicious, but the result of a complex new system being implemented shortly before the election. "You have to understand the magnitude of what we're doing," said one harried staffer at KLAS. Still, the stations are making millions from the ad bonanza, and the lack of manpower and a clear set of rules is resulting in a public record that's less transparent than it should be.

Also a matter of dispute: Exactly how inclusive the online files should be of correspondence between the station and ad purchasers. When Sunlight reported last week on a dispute between opposing groups on the issue of gay marriage in Maryland, uncovered by a document that turned up on the FCC database, a representative of the station involved insisted that the document didn't belong online. "This was a mistake," he said. Before the story even ran, the station had removed the file from the FCC database.

Updated 10/25/12 3:00 p.m.