Evidence collection in an anarchist society

Hey guys, I'm having a debate with someone about justice in an anarchist society.  But they've asked a question I don't know the answer to.  I hope you guys can help out.  We are talking about PDA's investigation a murder.  Here is the post:

Me:  They also can’t use coercion to force you into a little room to interrogate you for hours and hours, unlike the state.

Them:  They also would not be able to get DNA samples, fingerprint samples, or fiber samples from the accused clothes.  They could not search the accused's home for evidence.  They would not be able to question alibi witnesses should those witnesses not want to be interviewed. They would not even beable to compell the accused to stand trial.

Thanks!

But murder investigations were done long before there was DNA matching, or even fingerprints, etc.

Anyway, they probably could do those things, because there would be a prior agreement between the accused and his insurance or protection provider to provide evidence, etc., in the case he's accused of some crime, and perhaps between the witnesses and their providers to provide testimony, etc.  If they can't compel the accused to stand trial, they can still have the trial - the only one likely to get hurt if he chooses not to show up is the accused himself...why is that a problem?

I'd imagine voluntary contracts could be made with certain individuals; different groups of individuals with different schools of thoughts would have different ways of dealing with said activity, and if those individuals' rights are respected, the problem of the "taboo" is partially addressed.

How this would be fully solved, however, is interesting. 

If a community of people had a murder, and hired a PDA's to investigate, would not the community, or involved individuals if it isn't a community, create a binding contract for cooperation which would cease to function after said contract is fufilled?  Where individuals right's are respected, but so is the intention of finding out who violated one of those individuals rights, that of taking another life?

I would have no qualms with participating with such an investigation if a contract was made & honored.  If the contract were violated either way, the PDA would have to re-negotiate a new contract with the involved individuals, or if the individuals would have pay more for the services of the PDA (the former scenario assuming the PDA violated the contract, the later scenario involving the individuals involved that are not the PDA, violating the contract).

I think other's can come up with a better answer though, I know I'm missing something...