FERGUSON, Mo. -- Charges have been dropped against six activists who were arrested during a protest in Ferguson, just as their trial was about to begin. The case was expected to include allegations of police brutality, claims of missing evidence and discussions about the shortcomings of body cameras.

The charges had included property damage, resisting arrest, disorderly conduct and third-degree assault. The St. Louis Post-Dispatch reported Ferguson prosecutor Stephanie Karr dismissed the charges Thursday without explanation.

Later Thursday, the defendants filed a federal lawsuit alleging Ferguson destroyed evidence and violated the constitution.

Protests were common in Ferguson after the August 2014 fatal shooting by a white police officer of 18-year-old Michael Brown, who was black and unarmed. Officer Darren Wilson was not charged in his death, but the unrest prompted a U.S. Department of Justice report in March criticizing police and municipal court practices.

The Feb. 9 protest outside the Ferguson police station marked the six-month anniversary of Brown's death. Police said about 40 people participated in the protest, but video by protesters showed fewer than a dozen. The circumstances of the arrests were highlighted in the Justice Department report as an example of how Ferguson officers routinely violated First Amendment rights.

Heather De Mian, who uses a motorized wheelchair and was active in live-streaming several protests, faced the most serious allegation, an assault charge. Ferguson officials said De Mian, 46, blinded an officer with a light on her phone and struck him with it when he tried to push the phone away.

De Mian said the officer knocked her out of her chair without provocation. In a video taken by another activist, she is on the ground shouting, "They hit me in the face and knocked my glasses off."

Ferguson outfitted officers with body cameras after the Michael Brown case, and city policy calls for officers to wear them while on duty and retain the video in criminal cases. But months ago, in response to a public records request from the Post-Dispatch, the city said it did not have video from the camera worn by the officer who arrested De Mian.

City spokesman Jeff Small said the body cameras worn at the time had numerous technical problems.

"They just didn't work half the time," Small said. "We have since upgraded our cameras."

Also arrested at the February protest was independent filmmaker Christopher Phillips. His camera, valued at $20,000, was seized. When the device was returned, the connection pins on the memory card had been damaged, Phillips said.

Karr's notice the charges were being dropped was handwritten, and it said the charges could be refiled.

Information from: St. Louis Post-Dispatch, http://www.stltoday.com