I agree that LE doesn't have a suspect in mind, or if they do have a suspect, it is only a possible suspect, one whom can not be eliminated, but for whom there is no real evidence against. They do not have a "good suspect' whom they believe is the killer, for whom they are just "waiting for that one tip" before they arrest him.
If LE had a good suspect, by now, they would have examined every aspect of his life. They would have surveilled the suspect 24/7. They would know his friends and his employer. They would know his family. They would have interviewed them all. They would have obtained search warrants to examine his phone records and his internet history. They would have examined his driving history, his criminal history, his credit history, his bankruptcy history, his civil court history, put together a list of friends and acquaintances, and looked carefully at all of them, looking for weaknesses they could exploit to use against the suspect. With the help of the FBI, and IRS, they would have examined their history of federal and state tax returns. They would have looked at any welfare applications, food stamp applications and certifications, etc., auditing each and every one to ensure 100% compliance with all laws.
With all the laws existing in the US, nobody, not even a saint, could withstand this sort of scrutiny without LE learning of some laws being broken. If the suspect was a drug user, LE could set them up and catch them making a purchase. The likelihood is that the murderer of those two little girls would have a criminal history of lessor crimes increasing over time in frequency until he finally graduated to horrific murder.
Once the laws broken by the suspect are identified and documented, either a warrant for electronic surveillance or an arrest warrant for the identified crimes could be obtained. LE would then time the arrest to occur on a Friday evening, which would guarantee at the very least the suspect would be in custody until Monday morning. That would give LE 48 solid hours to interrogate the suspect about the murder and his whereabouts, plant an informant into their cell, etc. Possibly the DA could convince a judge to set bail high enough where the suspect would sit in jail longer than 48 hours.
Because none of the above has occurred, so far as we now, I feel that any suspects LE has are not strong suspects. That's why I believe LE does not know who the murderer is. If they knew who the murderer was, they would be moving heaven and earth to bring them to justice, if not for the murders, then for any other crimes LE could convict them of.
My prayers for justice for Libby and Abby.