Salt Lake County District Attorney Sim Gill said he and other officials are reviewing evidence and state statutes to determine the appropriate charges, which he expects to announce by midweek.
The teen, whose name is being withheld because he is a minor, is in juvenile detention on suspicion of aggravated assault. Authorities are considering additional charges since Portillo's death.
Legal analysts say the key factor for Gill to determine is intent.
Paul Cassell, a criminal law professor at the University of Utah, who is not involved in the case, said that from what he has seen it doesn't appear the teenager intended to kill Portillo, which means a murder charge is highly unlikely.
More probable is the lesser charge of "homicide by assault," which comes when an attack unintentionally causes death. The charge brings up to five years in prison for adults. Penalties can be less for juveniles.
"This appears to be a freak and tragic result of striking a referee," said Cassell, a former federal prosecutor and judge. "Clearly, that is a crime, but that is not going to be murder."
Tanya Lewis, a private attorney in Salt Lake City and legal analyst, who also is not involved in the case, said the charges should fall between homicide by assault and manslaughter, which is more serious.