In my opinion arresting someone who is a possible suspect in a murder investigation is a lawful arrest. The person arrested especially if they are treated with respect and “innocent until proven guilty”, would likely agree that it was necessary as well. Ask them to put their hands in front of them, cuff them, ask them to get in the car and explain that they need to verify they weren’t the culprit. Respect their time as much as possible and talk to them by basically saying I’m sorry we’re having to do this, we just need to get the witness to verify you weren’t the culprit, grab your prints and we promise we’ll have you back to your day as soon as we can. If you need to notify your spouse or contact your work my partner will be glad to help you contact them while we ride down to the station. Blah blah blah