On television, arrests appear incredibly simple. The police "talk" to the judge, get a warrant and arrest the "bad" guy. In reality, it can be that simple, but there are strict procedures the police must follow when they seek and execute an arrest warrant. This post will go over the various methods that police can use to obtain and execute a warrant.
The police can arrest people in three situations (that somewhat overlap). First, if a police officer personally observes you committing a crime, she may arrest you for that offense. After an arrest, the officer may also conduct a search which could lead to more evidence of different crimes and additional charges.
A police officer may also arrest you if she has "probably cause" to believe that a crime was committed. The officer must have a reasonable belief based on the facts and circumstances of the situation to arrest you. A typical example is an officer receives a report of a liquor store robbery. You match the description of the alleged criminal and you are observed in the vicinity of the liquor store. Those facts would likely constitute probable cause.
The final method that can result in an arrest is if a warrant is issued. The warrant must be issued by probable cause, but it is based on the gathering of evidence and under an investigation.
If the police fail to observe these procedures properly, the arrest may be unlawful.
If you believe that you were arrested under an unlawful warrant (or if you were arrested at all), then you may want to speak to an attorney. Be sure to go over in detail the circumstances of your arrest, they may be instrumental in establishing your defense. If your arrest was indeed unlawful, it could serve as grounds to exclude evidence or even get you released from custody.