One day after filing a federal lawsuit against the Jefferson County Sheriff's Department, the lead lawyer representing the family of slain Columbine High School teacher Dave Sanders charged the department with a coverup Thursday, alleging that officials have kept details of exactly what went on last April 20 from victims' families and the public.

"On at least one particular item, I affirmatively believe they were involved in a coverup," Peter Grenier said. "Absolutely, yes. A human being directly told us that," he said. "A human being affirmatively demonstrated a coverup."

He said the department has been extremely uncooperative, forcing the family to hire private detectives. "Our case is based on a lot of lengthy, expensive private investigation," he said. "No one from the sheriff's office would tell us anything."

The Sanders lawsuit contained a host of major new allegations, including claims that a sharpshooter had Dylan Klebold in his sights in the library, but his supervisors wouldn't allow him to act. It also contends the sharpshooter saw Klebold and Eric Harris commit suicide, and thus officers were aware the pair were dead three hours before Sanders died, but failed to rescue him.

Because of confidentiality agreements, Grenier said he was unable to be specific about the identity of the sharpshooter, the item covered up or the witness to it. "It will all come out in discovery," he said.

Earlier this week a judicial ruling granted the 15 families who were preparing lawsuits access to the sheriff's long-delayed "Final Report," but Grenier says the ruling came far too late to inform his filing. Neither the Sanders family nor its legal team has seen the report or the other newly available material. He expects to discover additional surprises within that data. "I hope to God there's something useful," he said.

Many victims' families have expressed anger about the long delay in releasing the final report on the Columbine investigation. Brian Rohrbough, father of Daniel Rohrbough, recently lamented on NBC News that he feared he would have to sue the sheriff's office to learn how his son died.

Sam Kamin, a law professor at the University of Denver, said the various lawsuits may well achieve their stated goal of bringing new evidence to the public. "If this gets far enough that they do discovery, more information about this may come to light," he said.

However, the families may be facing an uphill battle, he said, stressing that he had not read the suits and was offering a preliminary reaction based on knowing key points.

"It's going to be difficult," Kamin said. "It's going to be tough to ask a jury to say we know better than a SWAT team how to handle this situation."

The Sanders suit runs over 40 pages, with explicit, detailed information and allegations not previously aired.

The suit charges that the sheriff's office has mischaracterized the attack for the past year as a "hostage situation," when "the massacre was not at any point a 'hostage situation' under the clear meaning of the term set forth in Defendants' own manual."

It also alleges that the command team "knew before noon (although they have since tried to suggest otherwise) that there were two suspects, and that their names were Eric Harris and Dylan Klebold." The suit explains that the sheriff's deputy serving as security guard at the school knew both killers, and exchanged fire with them outside the school at the start of the attack.

A sharpshooter witnessed the killers' suicides, the suit claims, and therefore "the Command Defendants knew of their deaths to a virtual certainty by 12:30 p.m. at the latest."

That early knowledge of the killers' identities and their deaths changed the nature of the attack, and the expectations of the law enforcement response, the suit contends.

While the new allegations about the law enforcement response to the massacre have received the most attention, they are not the heart of the legal case. The family is claiming that Sanders' civil rights were violated "by prohibiting or impeding movement and then not doing anything to save him," Grenier explained. The suit hinges on the fact that the command team forced Sanders to remain inside Science Room 3, cut off several avenues of escape, repeatedly offered assurances that help would arrive within 15 minutes and then failed to respond as promised.

"Command Defendants exerted their control over everyone inside the school and over the hundreds of police, medical and rescue personnel massed nearby to prevent anyone from taking any action directed at rescuing or saving the life of Dave Sanders," the suit reads. It contends the command team prohibited students and teachers from breaking out a window and barred SWAT teams from attempting a surgical entry through the roof or one of many exterior doors almost directly beneath him.

"As a result ... Science Room 3 was the last [their emphasis] area in the building reached by SWAT teams ... and Dave Sanders was literally the last wounded person whom police or rescue personnel got to, even though Sanders was the only individual known to the Command Defendants throughout the critical several-hour period to be in urgent need of emergency lifesaving treatment."

The suit repeats what has long been common knowledge, that by noon law enforcement was aware of Sanders' situation and location via 911 calls and a large white message board reading "1 BLEEDING TO DEATH" facing toward the window. The department has contended that Sanders' wounds were severe enough that he would not have survived regardless, but the suit also challenges this claim.

The lawsuit by the family of Daniel Rohrbough and four other slain students also charges law enforcement teams with inaction on the day of the killings. "Although numerous deputies of the Sheriff Department were in a position to exchange gunshots with Harris and Klebold when they exited the school building, once again Defendants failed to either stop them outside the school or pursue the killers into the school," it says.

The suit paints even Harris and Klebold as perplexed by the inaction. "Several minutes after first entering the library, one of the killers went to the windows of the library and shot out several windows," it reads. "The Defendants made no response. Even the killers appeared puzzled and amazed at the Sheriff Department's non-response to their rampage."

But the heart of this lawsuit lies in its laying out a picture of two contradictory responses: the SWAT teams remaining outside to "secure the perimeter" while the 911 operators instructed students to remain inside because there was "help on the way."

"At the time the shooting began in the library, the students and teachers inside the library could have easily escaped to the outside of the building," the suit contends. "However, the 911 operator ... advised [teacher] Patti Nielson that Sheriff's deputies employed by Defendant Stone, paramedics and firemen would soon be arriving in the library and to stay there and on the line with her."

But the command team had full intention of carrying out a very different plan, the suit contends. After the killers entered the building, but before they entered the library, the command team "made a deliberate and intentional decision to limit the response of law enforcement officers at the school to 'securing the perimeter,'" it reads.

The result, it says, was the slaughter of 10 of the 13 victims murdered April 20.

Kamin stressed that a federal civil rights lawsuit requires evidence that law enforcement was not just ineffective but made the students worse off than if it had never shown up on the scene. The lawsuit addresses that issue directly: "By way of comparison, the children in the commons area (who were not encumbered by the 911 operator's commands) scattered to the various exits at the beginning of the incident and as a result none of these children were killed."

As with the Sanders case, this suit contends that the students' civil rights were violated because their own actions were impeded by law enforcement, who thereby assumed a special obligation to rescue them. "By telling the people in the school library to stay there and await law enforcement personnel ... [the Defendants] assumed an affirmative Constitutional obligation to protect those persons," it says. "By such conduct, the persons in the library were effectively in the custody of Defendants."

Kamin described one additional hurdle for this suit. "The department can't be sued unless it has a policy or pattern and practice that violates their civil rights." That explains the suit's otherwise surprising conclusion that both the SWAT teams and 911 operators acted exactly as they were trained to act.

It concludes by citing that contradictory training as the crucial final point in its claim. "As a result, on April 20, 1999, sheriff's deputies who chased Harris and Klebold into Columbine High School and 911 operators who instructed Patti Nielson and sheriff's dispatchers were not properly trained to handle the situation at Columbine High School."