I could see this being like a Kohberger or Tepe murders situation, where LE is aware of a suspect and keeping it close to their chest to not alert them ... but this situation is different because it's a medically fragile 84 y/o woman, who may still be alive, and each day that passes makes the situation seem more grim.
"Building a case and taking their time" just seems so much more dire when the victim could still be alive. I just hope they know more than they're letting on, and I hope there's a chance she's still alive.