I did some quick research to find out what is needed to get a search warrant. As most know, it has to have some basis and cannot be issues strictly based on a hunch.
I know this is somewhat general information, but here's what I found from Lawyers.com (I underlined a few things that might be most relevant for his case):
To get a warrant, agents need to:
1. Ask a neutral and detached magistrate or judge for a warrant. A "magistrate" is similar to a judge. He's usually an attorney or retired judge who's been hired by the court to handle certain matters to ease the judges' busy schedules. "Neutral and detached" means that the magistrate isn't involved in the case or the investigation and doesn't have a personal bias or "axe to grind" with the suspect. Persons who aren't neutral and detached include police officers, a district attorney, state attorney general or a justice of the peace who receives a fee for each warrant issued
2. Convince the magistrate that there is probable cause to search for and seize property. Generally, there's probable cause to search when there's a fair probability that contraband or evidence of a crime will be found in the place that the police want to search. The agents need to give the magistrate this information under oath or affirmation, that is, they need to swear that the information is true. Typically, they give the magistrate an affidavit - a written statement given under oath. However, a search warrant may be given when the agents give the magistrate this information orally.
3. Identify for the magistrate the particular and specific person or property to be searched and any person or property to be seized. This is called the particularity requirement. Generally, the street address of your house or the apartment number will be used; or a detailed description of you will be used if the warrant is for your body. It must also state that the warrant will be executed within 10 days and that it will be executed during the "daytime" - which generally is from 6 A.M. to 10 P.M. - unless the magistrate authorizes a different time.
So it's obvious that to get this warrant, law enforcement had to believe and convince a judge or magistrate - that they would find evidence of a crime at MR's house.
Now for my opinion - it's entirely possible that law enforcement found more remains than they say they have. There may be fiber evidence (i.e. carpet fibers) found, or evidence on one of the bones that shows an injury from a unique 'tool' that they're trying to locate. Lastly, I think it's possible that someone who visited the house may have seen something suspicious and told the police. (stepping off the soapbox).