Lawyer here. I don't practice criminal law anymore, but I did spend some time with the Federal Public Defender's office (defending individuals charged with Federal crimes).
The Federal government has the power to enact criminal statutes only for crimes with a nexus to a specific Federal power. This means that if the Federal government wants to prohibit certain conduct, that conduct must fall within the scope of legal authority expressly granted to the Federal government in the Constitution (like interstate commerce, for example) or an area of law that the Federal government has "preempted" (i.e, an area of law the Fed has taken away from the States--think like, airline regulation). Most Federal crimes stem from the Fed's power to regulate interstate commerce--so, things like drug trafficking and weapons trafficking. Then you have other unique Federal powers like immigration and national security which gives rise to immigration-related and terrorism-related crimes. Then there are other crimes you might not think of, for instance, maritime-related crimes, certain crimes that occur on Indian reservations, etc.
Other "every day" type crimes--local crimes like murder, burglary, etc--are left to the States. There is SOME overlap. For instance, the Federal government can prosecute a "local" crime that occurs on Federal land. So, the Federal government could, theoretically, prosecute a murder that occurred in, say, a national park. OR--here is a timely example--there is a big hubbub going on in Oregon right now surrounding a couple guys who were convicted in Federal court for committed Arson on federal land. Arson is USUALLY a state crime, but because it occurred on Federal land the feds had jurisdiction. I once worked on a murder case with the FPD that occurred on an Indian reservation. Still, these crimes are typically left to the States. The Feds are more concerned with "big fish" crimes--Mostly drug-related crimes, immigration-related crimes, and terrorism-related crimes (recall, for instance, the Boston Bomber was charged in FEDERAL court, not state court. He could have been charged in either, but because of the terrorism aspect, Federal court had jurisdiction. Also, the Federal government still uses the death penalty, whereas Massachusetts does not--so, I imagine that probably factored in to why he was charged by the Feds and not Mass. DAs).
However, the Feds can still prosecute what would otherwise be a "State" crime if its connected to a Federal crime. So, if the FBI arrests someone for importing illegal guns from Thailand or something and later finds out the same defendant committed a murder in relation to whatever criminal enterprise he/she was running, the Feds can also prosecute the murder. This is really common in big drug cases. I once worked on a case that was basically a big meth dealing scheme that spanned over three different States and there was a murder connected to the criminal conspiracy so the Feds included that as a charge.
If you want to read a really interesting case about the Federal government's ability to prosecute crimes, check out U.S. v Bond. It's a case from 2011. Basically, this woman found out that her husband had slept with her neighbor, so she put some harsh chemicals on her neighbor's front door and mailbox, and the neighbor got some chemical burns on her hands. Local police decided not to press charges because they saw it as a "domestic dispute," so the neighbor went to the feds. The Feds struggled to find a federal crime that they could use to prosecute the woman, so they ended up prosecuting her under a treaty the US entered agreeing to prohibit chemical warfare, basically. SPOILER ALERT: the Court reverses the woman's conviction because the treaty pretty obviously is intended to criminalize chemical weapons when used by like, a domestic terrorist or something, not a "local" crime like the one at issue.
Anyway, TL;DR: A Federal crime must be related to a Federal Power. There is some overlap, but the Feds would not have jurisdiction over a murder like Teresa Halbach's unless it was connected to some other Federal crime.