From the press conference by the Department of Justice, the Feds have shared their investigation results with the State of MA, and whether or not the State will take action to reverse SB's manner of death from suicide to homicide, is unknown. IMO, we should look to the State for first degree murder and/or any applicable State Homicide Law protecting viable, unborn fetuses. (Scott Peterson was charged by the State of CA, not the Feds).
However, specific to the federal Investigation of SB's murder by a Police Officer (i.e., under color of law), and subsequent Indictment by a Federal Grand Jury on 8/27/24, MF is being charged under the Federal Statute
18 U.S. Code § 1512 - Tampering with a witness, victim, or an informant-- not to be confused with 18 U.S. Code § 1111 - Murder.
(Please reference the 4 page Indictment linked upthread).
That said, relative to State law protecting viable fetuses, reportedly, the ME determined SB was
eight to 10 weeks pregnant when she was killed. It's unclear to OP if MA deems a 10 week fetus "viable" for the purpose of homicide charges if 24 weeks is used in MA for abortion.
MA statute provides that someone who kills an unborn baby
capable of living outside of the mother's body can be charged with homicide since that child is considered a person under state law...
"Killing is not murder unless a human being has been killed. A viable fetus is a human being under the law of homicide. A fetus is viable when there is a reasonable likelihood of the fetus's sustained survival outside the womb, with or without artificial support." Commonwealth v. Ronchi
www.law.cornell.edu