The answer is no, likely for two different reasons.
1. The year-and-a-day rule: At common law, a murder charge required that “the death transpired within a year and a day after the [injury]” (see Ball v. United States (1891), and this apparently remains the federal rule (see United States v. Chase (4th Cir. 1994)). Many states have apparently rejected this rule, given the changes in modern medicine that make it much easier to decide whether an old injury helped cause a death; but though the Supreme Court in Rogers v. Tennessee (2001) held that a court could retroactively reject the rule without violating the Ex Post Facto Clause (which applies only to legislative changes to legal rules) and the Due Process Clause, any such retroactive rejection of the year-and-a-day rule seems unlikely in this case (given that for the rule to be reversed the case would likely need to go up to the Supreme Court, and that in any event the rule had been applied relatively recently, in Chase).
UPDATE: But wait! Hans von Spakovsky points out that D.C.’s highest court rejected the year-and-a-day-rule in United States v. Jackson (1987). I at first didn’t check the D.C. precedents, focusing just on precedents from the normal article III federal system, since Hinckley was tried in federal district court. But Hinckley was tried for violating D.C. law as to the shooting of Brady (though the case was in federal district court because of the federal charges as to the shooting of President Reagan and the Secret Service agent), and by hypothesis would be retried for violating D.C. law. It is D.C. law that would apply here.
Yet Jackson was a 1987 case, decided after the Hinckley shooting. It expressly stated that, as of 1987, the year-and-a-day rule was in effect under D.C. law:
The common law of the District of Columbia encompasses all common law in force in Maryland in 1801, unless expressly repealed or modified. [Citations omitted throughout. -EV] In 1776, Maryland adopted the common law of England as it then existed. Therefore, we look to early Maryland law to resolve the question whether the year and a day rule is law in the District of Columbia.
The Court of Appeals of Maryland has held [in 1974 and in 1985] that the year and a day rule is part of the common law of Maryland because it was part of the English common law in 1776. A division of this court (in an opinion subsequently vacated for reasons not pertinent here) has unanimously concluded [in 1979] that the rule retains its viability in the District of Columbia. In re J.N., Jr., 406 A.2d 1275, 1283 (D.C. 1979).
We follow this reasoning in concluding that the common law year and a day rule is today the law in the District of Columbia.
And the D.C. court (in Jackson) expressly held (based on the Ex Post Facto Clause) that its abrogation of the rule would thus apply only prospectively, to prosecutions after that decision.
What should the effect of that be? On one hand, as I noted above, the U.S. Supreme Court in Rogers held that Tennessee courts could change the rule without violating the Ex Post Facto Clause, so that might undermines Jackson‘s prospectivity-only reasoning. This might mean that the prospective-only rule can be retroactively changed to a retroactivity-OK rule.
But on the other hand, Jackson expressly stated — in a discussion of substantive D.C. law, not of the Ex Post Facto Clause — that, as of 1987, the year and a day rule was still the law in D.C. And the U.S. Supreme Court in Rogers stressed that “perhaps [the] most important" part of the reasons for its acceptance of the Tennessee court’s retroactive rejection of the rule was that, “at the time of petitioner’s crime the year and a day rule had only the most tenuous foothold as part of the criminal law of the State of Tennessee." The D.C. Court of Appeals’ analysis in Jackson makes clear that “at the time of [Hinckley’s crime] the year and a day rule” had much more than a “tenuous foothold” as part of D.C. criminal law.
So I think that, if Hinckley were tried now for murder under D.C. law (which he couldn’t be, for the independent reasons below, but let’s set those aside for now), he would be tried under D.C. law as it existed in 1981, at the time of the shooting. And, given the D.C. highest court’s analysis of D.C. law in Jackson, that as-of-1981 law would include the year-and-a-day rule, which would make Hinckley substantively not guilty of the crime of murder.
2. Double jeopardy and collateral estoppel: But say the year-and-a-day rule didn’t apply here — hasn’t Hinckley already been tried for the shooting, and doesn’t the Double Jeopardy Clause stop him from being retried? The answer is yes, but in a circuitous way.
a. It’s OK to try someone for murder, even if he’s already been convicted of the attempted murder: If Hinckley had been tried for attempted murder and found guilty when Brady was still alive, he could be tried for murder after Brady died. Indeed, this is pretty much what happened in Diaz v. United States (1912), and this remains the law today, see Garrett v. United States (1985):
In Diaz v. United States, 223 U.S. 442 (1912), the Court had before it an initial prosecution for assault and battery, followed by a prosecution for homicide when the victim eventually died from injuries inflicted in the course of the assault. The Court rejected the defendant’s claim of double jeopardy, holding that the two were not the “same offense” ….
b. It’s sometimes OK to try someone for murder, even if he’s already been acquitted of the attempted murder: Say someone (not as in the Hinckley case) acted extremely recklessly and injured someone else as a result. If the reckless person were tried for attempted murder, he’d be acquitted, because attempted murder generally requires a conscious purpose to kill, not just reckless endangerment.
But say the injured person then dies. The defendant could then be retried for murder, and convicted, because actual murder (not attempted murder) can happen even if the defendant didn’t have a conscious purpose to kill, so long as he was extremely reckless.
c. But the jury’s conclusion that Hinckley was insane is now binding on the government, and thus precludes a retrial for murder: Under the “collateral estoppel” doctrine (Ashe v. Swenson (1970)), “when an issue of ultimate fact has once been determined by a valid and final judgment, that issue cannot again be litigated between the same parties in any future lawsuit.” That’s most commonly applied in civil cases, but it also applies in criminal cases, against the government (so long as it’s the same government involved in both cases).
The jury determined by a valid and final judgment that Hinckley was insane, and thus couldn’t be liable for attempted murder. This judgment is binding on the government, and since the insanity defense applies the same way for murder as for attempted murder, it means that Hinckley would now be conclusively presumed to have been insane for purposes of any murder prosecution as well. He would have an ironclad defense to the murder charge, and thus any case against him couldn’t proceed. For a similar case, see United States v. Oppenheimer (1916), though there the defense was the statute of limitations rather than insanity.
(Note that the federal insanity defense has been considerably narrowed since the Hinckley trial — indeed, as a result of the Hinckley trial. But this legislative narrowing can’t be retroactively applied, given the Ex Post Facto Clause.)
3. UPDATE: What about the possibility that Hinckley would be prosecuted by Virginia, where the crime of murder was finally completed, since that’s where James Brady died? Some commenters asked me that; I’ve written a follow-up post discussing this in some detail, but the short version is that a Virginia prosecution is also probably not an option.
So no retrial for Hinckley, despite the medical examiner’s conclusion, and even if that conclusion could be proved beyond a reasonable doubt. Or so it seems to me, based on my general knowledge of the field, and my research this afternoon; please let me know if there are some legal arguments that I’ve missed here.
UPDATE: Some people have asked whether the “dual sovereignty” doctrine would allow Hinckley to be tried in the D.C. court system (starting in the D.C. Superior Court), given that the earlier trial was in the federal court system (the D.C. District Court). The answer is no, because D.C. and the federal government — as opposed to state governments and the federal government — are the same sovereign. Because of this, “successive D.C. and federal prosecutions for the same conduct are subject to the bar on double jeopardy, whereas successive state and federal prosecutions are not.” United States v. Mills (D.C. Cir. 1992).