The Boston Police have evidently tweeted out a denial that there has been an “arrest” in the Marathon bombing investigation. Obviously, there is much confusion at the moment, with different sources who have varying levels of direct information providing conflicting accounts. Might be useful to bear three things in mind.
First, when someone won’t put his name on an assertion, be suspicious — such people tend to have a tenuous relation to what is actually happening in the investigation. They may not intend to mislead anyone, but they may be out of the information loop, or at least behind the information curve.
Second, there is a technical difference between “arrest” and “custody.” It is entirely possible that someone could have voluntarily submitted to an interview or could have been temporarily detained for questioning. Such a person might not have been formally charged, and it might be ambiguous whether he is free to leave the place where police are questioning him. Freedom to leave is the usual test of whether someone is under arrest; and, of course, if a formal complaint has been filed and an arrest warrant executed, that removes any doubt. But it is entirely possible for the police to have someone in custody — voluntarily or (in the tricky area of temporary detention) even involuntarily — and for that person not to be formally under arrest.
Third, there is the opposite situation. Often, the police reach a point in their investigation where they’ve identified a suspect and believe they have probable cause to make an arrest, but that person is not in custody — in fact, his whereabouts may be unknown. In such a situation, a prosecutor will file a formal complaint and persuade a judge to issue an arrest warrant. Unexecuted arrest warrants are usually kept under seal with the court until the arrest is actually made at some later time. That is, there are many times when an arrest warrant has been ordered but no actual arrest has yet been made.
I have no reliable information on what is actually going on in the investigation. I merely point these things out to make the point that most reporters and pundits are not lawyers, and even many law-enforcement agents have a limited understanding of the technicalities discussed above. It is thus easy to understand why, once terms like “arrest” and “custody” start being thrown around by people who do not know what is actually going on in an investigation, a lot of misinformation ends up being reported.