It's my understanding that an officer must witness a misdemeanor to site. It the offense is a felony, the officer can take a report from a witness and arrest the suspect. One of our resident police officers can clarify this.
Here's an incident that we could compare this to. A man walks into a SevenEleven and bradishes a gun, demanding money. Witnesses see the event. They give an account of the incident, a description and a license plate number. An officer a few blocks away sees the perp in the get-away car driving down the street, effects a stop and makes an arrest.
A person doesn't have to be a Department of Justice attorney, or a local prosecutor to effect an arrest. A private citizen can make an arrest if he feels there are grounds. He does expose himself to charges if he abuses this concept.
An attorney for the Department of Justice, is considered to be an officer of the court. Their testimony should be above reproach.
If a person filing a report lies, they are subject to the laws applicable for such an offense. Clearly in this case, a false report was issued. If an officer of the court were found guilty of having done this, it might affect their licensure. It might also merit them serious jail time. Filing a false report regarding a civil rights violation, is a serious offense.
To answer your quetions and points raised.
Anyone can make an arrest without a warrant for justifiable cause.
The police made an arrest based on the report of a witness to what the witness claimed was a crime. Evidently there was a judgement call by the officers that the witnesses story seemed reasonable enough to have been taken serious.
Every citizen can give a report of what they observed. DOJ attornies do not have authority in local matters. But they do have as much right as anyone else to bear wtiness. And I think you're kidding yourself if you think a police officer wouldn't take their position as indication of their veracity.
According to news items posted here, these lawyers did not file a formal complaint, they merely walked up to officers, flashed their DOJ Ids, and started ordering arrests.
The local police were CLEARLY not using their own judgement, and even let the DOJ lawyers cow them into making arrests by citing reading from the Bible as hate speach. (while totally ignoring the fact that the DOJ lawyers were engated in a hate crime themselves).