“There’s an imbalance in almost all systems of the world between what law enforcement is allowed to do, versus publicly funded research, versus private companies,” said Barbara Prainsack, a professor at the University of Vienna who studies the regulation of DNA technology in medicine and forensics.

While some countries, like Germany, have an approved green list of technologies and forms of evidence that law enforcement agencies can use, it’s exactly the reverse in the United States.

“It’s a total wild west, a free for all,” said Ms. Murphy, the N.Y.U. law professor. “The understanding is police can sort of do whatever they want unless it’s explicitly prohibited.”

Often, the public and other branches of government learn that law enforcement officials have adopted a new technique only at a news conference announcing an arrest, Ms. Murphy said. She pointed specifically to the arrest of Joseph James DeAngelo, the Golden State Killer, which the police credited to the use of genetic genealogy — entering crime scene DNA into family history databases and triangulating a criminal’s identity based on distant cousins. In those high-profile cases, she said, law enforcement personnel rely “on the good will they engender when they do use the technology for really positive uses.” Other uses might not be disclosed.

Safeguards against misusing a new technology like eDNA rely on the courts, where experts say the track record is poor.