CHICAGO (CBS) — Thirty days before Mayor Rahm Emanuel faced the biggest test of his political life, City Hall received a blockbuster letter from attorneys representing Laquan McDonald, accusing police of making a series of false statements and even intimidating at least one witness who saw the Chicago teenager fatally shot by Officer Jason Van Dyke.

That March 6, 2015 letter from attorney Jeffrey Neslund, quickly set in motion negotiations that led to an out-of-court legal settlement, reached just more than a month later — an astonishingly quick resolution for any legal matter.

After a bruising campaign, Emanuel won re-election on April 7, as those settlement talks quietly neared conclusion.

In the letter, which was part of about 1,400 emails released by the city on New Year’s Eve, Neslund informs Deputy Corporation Counsel Thomas Platt that police dash cam video “confirms that Mr. McDonald did not lunge toward police” which is “contrary to the false statements the city allowed the [police union] spokesman to spin to the media.”

Neslund then prophetically warned Platt that “the graphic dash cam video will have a powerful impact on any jury and the Chicago community as a whole. This case will undoubtedly bring a microscope of national attention to the shooting itself as well as the City’s pattern, practice and procedures of rubber-stamping fatal police shootings of African-Americans as ‘justified.’ ”

Indeed, the release of the video sparked national attention on Chicago police practices, led to the firing of Police Supt. Garry McCarthy, a series of policy changes–and even an apology–from Emanuel, and multiple protests through downtown Chicago.

The letter alleges that police officers tried to get one witness–who was so appalled at the incident that she screamed at Van Dyke to “stop shooting”–to change her account.

The witness, whose name was redacted in the documents, was transported to the police station “where she was held against her will and intensively questioned for over six hours.”

Neslund’s letter accuses detectives of repeatedly attempting to get the witness to change her statement because, they said, her story “did not match the video.”

She was finally released at 4 a.m. after demanding a lawyer.

McDonald, who was carrying a knife, was shot 16 times on Oct. 20, 2014 near 40th Street and Pulaski Road, as officers were responding a report of a person breaking into cars.

“There is no plausible justification for such an excessive use of deadly force. … Yet officer Van Dyke chose to empty his 9 mm pistol into the body of Laquan McDonald rather than allow other officers to employ non lethal force.”

Van Dyke has since been charged with first-degree murder and has pleaded not guilty.

The piles of documents also show city officials began corresponding among themselves about the shooting.

Emails show officials from the mayor’s office and that of the Independent Police Review Authority were in contact about the case. As CBS 2’s Dana Kozlov reports, that raises the question: Just how independent is the Independent Police Review Authority?

Mayoral spokesperson Adam Collins says inter-office communication is not unusual.  He adds, “The documents show there was no attempt to cover anything up by this office.”

Contributing: web editor John Dodge, political producer Ed Marshall, reporter Dana Kozlov