This image by the U.S. National Cancer Institute shows the 46 human chromosomes, where DNA resides. (National Cancer Institute/AP)

Maryland’s high court has thrust the state into a national debate over how police collect and use DNA evidence, issuing its third decision in as many years governing when law enforcement practices cross the line to unconstitutional invasions of privacy.

In a 5-to-2 opinion, the state’s Court of Appeals ruled this past week that police can take a DNA sample from a voluntary donor in an investigation and use the resulting profile to investigate other crimes without a court order, as long as the donor did not expressly limit his or her consent.

Dissenting judges said the decision will have a chilling effect on victims, relatives or others who volunteer their DNA to help police, leaving them with fewer legal protections over the use of their genetic material than those enjoyed by people convicted of a serious crime.

The opinion follows a string of state decisions that have probed the legality of expanding DNA collection beyond convicted offenders, the growing power of technology to test tiny amounts of DNA naturally shed by people without a body search, and whether unrestricted testing of lawfully obtained DNA amounts to a form of lifetime genetic surveillance.

“In the physical world, you know what you’re consenting to if police say, ‘Can I search your car?’ ” said George Washington University law professor Orin Kerr, a former federal prosecutor and expert on criminal procedure and technology. “With DNA and with computers, people might not know what consent means, how much information is contained there and what can be done with it.”

Kerr added, “The real question is, are there any limits on government’s ability to scrape DNA from whatever you touch, test it and run it in a database at any time?”

The latest decision involved George Varriale, a homeless man who lived in a wooded area of Glen Burnie, who in 2012 agreed to let Anne Arundel County police take a cheek swab for DNA to investigate a rape.

Varriale was cleared in that case. But on her own initiative, the county crime lab’s DNA administrator uploaded his genetic profile into state and county databases with the DNA of arrestees and offenders. The databases conduct automated weekly searches. Varriale was matched to a 2008 burglary, entered a conditional guilty plea and appealed.

In a 28-page opinion, Court of Appeals Judge Clayton Greene Jr. said DNA information used by police comes from “junk DNA” segments that do not reveal genetic traits and do not present a privacy threat.

“DNA profiles are like fingerprints, which police routinely catalog and compare in the course of criminal investigations,” Greene wrote. “Absent an express limitation placed on the use or storage of the DNA evidence by Varriale, the State, or by law, we cannot conclude that it was unreasonable for the State to maintain and utilize Varriale’s DNA for subsequent unrelated investigation.”

In a dissent, Judges Glenn Harrell and Sally D. Adkins warned that the decision will discourage consenting DNA donors and that “the time may be nigh” for new regulations or legislation to protect them.

That is because when the General Assembly passed Maryland’s DNA Collection Act in 1994, the rules for collecting, retaining and in some cases expunging DNA applied to certain convicts and — as amended in 2008 — arrestees, but not to anyone else, the judges said.

Voluntary donors “will face a certain knowledge that, even if not suspected or convicted of a crime, the police can, and will, hold on to their DNA profile forever, and may compare it at any time for any or no articulable reason,” Harrell wrote.

Harrell, now retired, also criticized the court’s majority for concluding that an ordinary person could easily understand a police form to prohibit the use of his or her DNA to investigate other crimes.

“This is beyond the ken of a typical lay person, even one who is not a homeless person living in the woods,” Harrell wrote, adding that the majority is “a hop, skip and a jump” away from endorsing police fishing expeditions of victims, because DNA databases are used not only like fingerprints to identify people, but to investigate them for crimes.

The Varriale case is the latest in Maryland interpreting DNA collection under the U.S. Constitution’s Fourth Amendment ban on illegal searches. In 2014, the Supreme Court in a 5-to-4 vote upheld the constitutionality of Maryland’s DNA collection law for arrestees, overturning the state high court’s prohibition a year earlier.

Separately, the Supreme Court in March declined to take up the appeal of a 2014 decision by the Maryland high court, which upheld the constitutionality of the warrantless collection of DNA from a man not under arrest without his consent.

In that case, Glenn J. Raynor came to a police station to answer questions in a 2006 rape in Harford County. Raynor offered to give a DNA sample if police agreed to destroy it after the investigation, but police declined.

After he left, police obtained his DNA from sweat left on his chair. Raynor was convicted and sentenced to 100 years in prison.

On appeal, Maryland’s court ruled 4 to 3 that Raynor “abandoned” his DNA and had no expectation of privacy.

Jennifer Lynch, senior staff attorney with the Electronic Frontier Foundation, a nonprofit civil liberties group based in San Francisco, said the Raynor and Varriale decisions pose a “Catch-22” situation.

The Varriale opinion says that if someone volunteers to give police a genetic sample from a cheek swab in one investigation but doesn’t want it used in other cases, he or she must expressly seek to limit its use, Lynch said.

But according to the Raynor case, if that happens, then police can reject those terms and acquire the DNA on their own because people constantly shed microscopic quantities of DNA in blood, sweat, saliva and skin cells, Lynch said.

“It really puts us in a terrible position because we have no ability to control what happens to our DNA,” Lynch said. “We can’t live in a hazmat suit.”

Jennifer L. Mnookin, dean of the UCLA School of Law, said the Varriale case also presents a powerful example of DNA “database creep.”

Since 1994, Maryland law has required that sex offenders submit a DNA sample to the state database. That was later expanded to include anyone convicted of felonies and some theft-related offenses and, in 2009, anyone arrested or charged with violent crimes or burglary, regardless of the outcome of the case.

State police say they have 116,330 convicted offender profiles in Maryland’s DNA database and 28,828 samples from people who have been arrested or charged with offenses.

But Maryland State Police spokesman Gregory M. Shipley said state and local police also keep a “suspect” database of individuals who have not been arrested or charged, including 3,378 profiles in the State Police system and 5,511 in the system held by police in Baltimore and in Montgomery, Prince George’s, Anne Arundel and Baltimore counties, for a total of 8,889.

Such profiles from victims, relatives, excluded suspects and other consent-based samples cannot be uploaded to the national FBI database, which includes more than 11 million offender profiles and 1.9 million arrestee profiles.

But they present a case study of “a shadow system” of lightly regulated state and local databases around the country whose number and size have mushroomed, said Stephen B. Mercer, chief attorney of the forensics division of the Maryland Office of the Public Defender and Varriale’s lawyer.

“This is the next wave of DNA database expansion,” Mercer said in a paper he co-wrote in 2014 for New York University’s Annual Survey of American Law. He added, “Unfortunately . . . [the] consequences could very well endanger public confidence in the core mission of the regulated national DNA databank.”

Shipley said DNA evidence and analysis remain invaluable for police to help identify the guilty and exonerate the innocent, adding that Maryland’s DNA database scored its 4,000th match in May.