Jury selection for Fall River, Mass., trial of Aaron Hernandez on first-degree murder charges began on Friday, and already this case has had more ups and downs than a roller coaster. The case involves the June 2013 murder of Hernandez's friend, semi-pro football player Odin Lloyd, in North Attleboro, Mass. Hernandez, who at the time of the murder was a tight end for the New England Patriots football team, is accused of orchestrating the murder with the assistance of Carlos Ortiz and Ernest Wallace, who are facing similar charges. Although the prosecution has yet to set forth a motive for the murder, the prevailing theory is Hernandez no longer trusted Lloyd, whom court records suggest may have had knowledge about a 2012 Boston double homicide in which Hernandez is a suspect.

The trial is expected to take months and cameras will be allowed in the courtroom. There have been a number of legal milestones in the case already. Multiple motions have been decided, and Judge Susan Garsh has precluded much of the evidence that many feel would have been the most damaging to Hernandez's defense.

For example, on June 17, 2013, Lloyd sent his sister a series of text messages that read, "U see who I'm with?" followed by, "NFL," "just so you know."

Prosecutors allege that within an hour of sending the texts, Hernandez had shot and killed Lloyd. They had planned to use Lloyd's texts to prove Hernandez's guilt in the matter to a jury, beyond a reasonable doubt, but the judge has disallowed the presentation of the texts.

Every piece of evidence precluded from the prosecution's case weakens it and such a ruling certainly hurts the prosecution's case, but it's a ruling that is extremely appropriate under the law.

If you watch legal shows on television, you have heard the term "hearsay," which is loosely defined as an out of court statement, offered into evidence to prove the truth of a matter that has been asserted. (See Massachusetts Rule of Evidence 801[c]). But the application what constitutes hearsay in court is complicated, and there are numerous exceptions within the definition.

Generally speaking, however, hearsay is not admissible most of the time - and for good reason. According to the law, lawyers must be granted the opportunity to cross-examine those who are presenting evidence to the jury. They must be allowed to test its credibility and reliability. But because Lloyd is dead - and therefore unavailable to testify - Hernandez's attorneys cannot question him about what he meant to convey with his texts. Specifically, they can't ask him about whether he was attempting to identify his killer. Therefore, according to the law, the jury cannot be permitted hear such statements. (See Massachusetts Rule of Evidence 802).

Of course, there are exceptions to hearsay rules. The one prosecutors tried to argue in this case is called the "dying declaration" exception. Under the "dying declaration" exception, hearsay evidence is admissible "if the statement is made by one with a belief of imminent death and who died shortly thereafter, concerning the cause of death." (See Massachusetts Rule of Evidence 804[b][2]).

Statements like "Jake is about to kill me" would usually fall under this exception, which is why the prosecution argued that Lloyd's texts were equivalent to his declaration that "Hernandez is about to kill me." However, the judge has ruled such an argument is just too speculative. The texts can be just as easily interpreted as Lloyd bragging about his friendship with Hernandez. The fact that Lloyd's sister responded "LOL" to one of these texts may support the judge's ruling regarding alternate interpretations.  

The judge has excluded additional evidence, including the prosecution's desire to reference that Hernandez is charged with a 2012 double murder in Boston. In general, other crimes are not admissible in proving the character of a person but can be used to show motive. (See Massachusetts Rule of Evidence 404[b]).

The prosecution has argued Lloyd knew about the other crime and that given Hernandez's desire to kill him, the other crime should be allowed as evidence. But Judge Garsh also found that argument to be speculative and subsequently disallowed it.

Hernandez's defense attorneys were likely very satisfied with these rulings because their job has become a whole lot easier as a result. But a great deal of evidence remains. For instance, prosecutors will be able to present surveillance footage of Hernandez at his home, holding a gun on that night. They'll also present footage of Hernandez's car leaving the industrial park where Lloyd was shot and returning to Hernandez's home soon after the killing. These are significant pieces of circumstantial evidence and the jury will likely give them considerable weight.  

Now, though, there may have been a major breakthrough for the prosecution in this case. A number of meetings held last week are leading some to believe that Hernandez's fiancee, Shayanna Jenkins, may be ready to make a deal with the prosecution. Jenkins has been charged with perjury, and prosecutors allege she lied to the grand jury in this case. If she is convicted of perjury, she could face up to 20 years in prison. If she enters into a deal with the prosecution, she could get immunity for her testimony and avoid any jail time. This would be the defense team's biggest nightmare. All of its hard work and all of the evidence it has successfully had precluded will mean next to nothing if Hernandez's fiancee testifies against him.  

So far, this case has reminded us that trials are not over until a verdict is read. Every trial is a series of ups and downs for both sides, and small details often make a big difference in outcomes. If the Aaron Hernandez trial is looked upon as a race, we are now only at the starting line. Bear in mind that this is a marathon - not a sprint - and it'll certainly be one to watch.

Heather Hansen is a partner in the O'Brien and Ryan law firm who has been named one of the 50 top female lawyers in the state by Pennsylvania Super Lawyers. Heather is also a national television and radio legal analyst and journalist who has appeared extensively on CBS News, Fox News, Fox Business Channel, Fox.com, CNN, HLN and Sirius XM radio. Her writing has appeared in Law360 and she has co-authored two chapters in medical texts regarding medical malpractice litigation. Follow her on Twitter at @imheatherhansen.