Darren Wilson, Michael Brown and the shooting scene on Canfield Drive in Ferguson. (AP Photo)

Former Officer Darren Wilson of the Ferguson, Mo., Police Department destroyed potentially crucial evidence shortly after fatally shooting Michael Brown, the slain teen’s family alleges in a lawsuit filed Thursday.

The unarmed 18-year-old was killed last August during a physical altercation with Wilson. A state investigation and a federal criminal civil rights probe resulted in no charges against Wilson.

But attorneys for the Brown family say the investigation was bungled from the beginning, when police supervisors allowed Wilson to leave the crime scene unescorted and return to the Ferguson Police Department, where he washed his hands and stored his weapon.

Officer Darren Wilson's .40-caliber service weapon. (St. Louis Co. Prosecutor)

“Defendant Wilson returned to the police station and began destroying evidence and interfering with the investigation,” the lawsuit states.

According to his account, Wilson said he spotted blood on both hands while driving back and wanted to get clean it off for safety reasons.

“I immediately go to the bathroom,” he told a grand jury in September. “Just from everything that we have always been taught about blood, you don’t want it on you, you don’t touch it, you don’t come in contact with it.”

No photographs were taken before he washed his hands twice.

A few minutes later, Wilson put on rubber gloves, removed the one bullet he hadn't fired at Brown from his gun and put the weapon in an evidence bag. Prosecutors overseeing the grand jury asked him why he wore gloves to store the gun.

“To preserve any evidence on there,” he testified. “I knew his DNA was on that gun.”

David Klinger, an expert on police shootings, told the Washington Post in November that time-tested investigative procedures were not followed in the critical moments following Brown’s death.

“An officer driving himself back? Wrong. An officer booking his own gun into evidence? Wrong,” said Klinger, a University of Missouri-St. Louis professor who is also a former police officer.

The Brown family's lawsuit alleges that Wilson’s actions amounted to tampering with “critical pieces of evidence by destroying potential gun residue on his hands, blood, and/or DNA evidence, as well as compromising the integrity of his weapon, which had significantly probative evidentiary value.”

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The officer’s gun is crucial to the case. Wilson, 28, said that Brown punched him in the face and tried to take his gun during their altercation. Because of his testimony and physical evidence at his patrol car, two investigations concluded that the officer had been in reasonable fear of his life to use deadly force.

Click image to read entire lawsuit.

According to a Department of Justice report, autopsy results, bullet trajectory, skin from Brown’s palm on the outside of the SUV door and Brown’s DNA on the inside of the driver’s door corroborate Wilson’s account that during the struggle, Brown used his right hand to grab and attempt to control Wilson’s gun.

But the Brown family continues to maintain that Wilson was the agitator.

They allege that Wilson started the physical confrontation by grabbing Brown’s shirt and neck, and then drawing his gun and pointing it at Brown.

“[Michael Brown] desperately attempted to break free from Defendant Wilson’s unlawful encounter in an effort to protect himself from further physical force by Defendant Wilson, including being shot,” attorneys write in the lawsuit.

Two shots were fired from the patrol car, one grazing Brown’s hand. The teen then ran down the street, where Wilson fired 10 more times from outside the vehicle. Autopsies ruled that Brown, who died at the scene, was struck at least six times.

Once back at the police station, the lawsuit alleges, some of Wilson’s actions were observed by Officer Barbara Spradling, a former supervisor to whom he was engaged at the time of the shooting. The couple has since married.

The Brown attorneys fault the city of Ferguson for failing to conduct a proper, fair, and impartial investigation.

“Defendant City ratified Defendant Wilson’s misconduct by failing to reprimand him for (1) Interfering with an internal investigation, (2) tampering with evidence, and/or (3) destroying evidence, as well as not reprimanding his fiancé for failing to intervene due to her having witnessed the destruction of and/or tampering with physical evidence.”

The wrongful-death suit seeks unspecified punitive and compensatory damages, including medical treatment for psychological damages. It also asks the court to order the Ferguson police to quit using patrol techniques that allegedly demean black residents. In addition, the suit requests that the court appoint a compliance monitor to oversee the Police Department’s use-of-force practices for five years or until all officers are effectively trained in constitutional requirements.

“I don’t have any comment,” City Attorney Stephanie Karr said in an email to Yahoo News.

An attorney for Darren Wilson did not immediately respond to a message seeking comment. Former Ferguson Police Chief Tom Jackson, also a defendant, told the Associated Press that he had not had a chance to review the allegations.

Brown family attorney Benjamin Crump told reporters in St. Louis that new forensic evidence planned to be brought forth for the civil case would shed light on the controversial shooting.

“The narrative of the law enforcement all across the country for shooting unarmed people of color is the same: that they had no other choice,” Crump said. “But time and time again, the objective evidence contradicts the standard police narrative.”

Jason Sickles is a reporter for Yahoo News. Follow him on Twitter (@jasonsickles).