A grand jury will soon weigh cops’ case against two teen suspects in the Morningside Park murder of Tessa Majors, with an eye towards a possible indictment, law-enforcement sources have said.

But, what authorities once hoped would be the most damning piece of evidence in their arsenal — DNA tying the suspected 14-year-old killer to the tragic Barnard College freshman — is actually far from conclusive, sources told The Post.

DNA evidence ostensibly connecting the youth to the fatal knifing has yielded a 1-2 million likelihood ratio, sources said — a far cry from the 5-billion figure investigators prefer to have anchoring a murder case.

“That’s a very low match. That means some of the stabber’s DNA structure matches some of the DNA” recovered as evidence, said one NYPD insider. “The 1-2 million [figure] is not conclusive that the stabber is the one who left his DNA at that crime scene.”

The insider defined a likelihood ratio as “the certainty that the suspect is in fact the donor of the DNA at the crime scene,” relative to all other people in the world. A 1-million ratio suggests that one in 1 million people is a match — meaning more than 7,000 people in the world’s 7.7 billion population could appear to be a match to the sample collected. A 5-billion ratio, on the other hand, means that there is almost no one else on earth with that DNA profile.

Factors including exposure to the elements — and the DNA of others — can degrade the sample and cause lower likelihood ratios, the source explained.

Two other law-enforcement sources concurred that the DNA evidence is inconclusive, and while authorities still intend to present it for the consideration of a grand jury, it will likely be less as a smoking gun and more as a facet of a meticulously assembled case.

“There’s no slam dunk, but if you’re a prosecutor you want to go in with a significant statistic that tells the jury the rarity” of the DNA, explained Lawrence Kobilinsky, a professor of forensic science at the John Jay College of Criminal Justice.

But, Kobilinsky contended, the results of the testing are hardly a death knell for the case.

“They want to say it’s unique,” he said. But “There are plenty of people convicted without a unique match. It’s nice if you’ve got five billion, but not needed.”

Majors, 18, was cutting through Morningside Park on a dark December afternoon when she was cornered by trio of assailants, one of them waving a knife, authorities have said.

The group tried to rob the musician and aspiring journalist, but she fought back, chomping down on the armed suspect’s finger as they tussled, sources have said.

Detectives have theorized that that desperate bid for survival enraged the mugger, who stabbed Majors multiple times — and with such ferocity that feathers flew from the lining of her winter coat.

Majors was found unresponsive at the top of a flight of stairs leading out of the park, having dragged herself there with the last of her strength.

The chaotic and decidedly unsophisticated nature of the murder — a knife-point stick-up gone horribly awry, apparently at the hands of three middle-schoolers — belied the deliberate nature by which cops built their case.

Save one grainy surveillance video caught on a camera outside the park — which rendered the run-in a shadow play, clear enough to convey the gist but not to reveal any faces — there was no known footage.

Nor were there any known direct eyewitnesses, or at least any impartial ones.

Zyairr Davis, 13, was the first to be charged in the case, accused by prosecutors in family court of felony murder.

He has purportedly admitted to cops a role in robbing Majors, but maintained that one of his two 14-year-old pals plunged the knife into their victim.

But “You cannot get a prosecution off co-defendant statements,” said one source, because Davis has everything to gain by pointing the finger at one of his alleged accomplices.

As for those two teens, cops have brought each of them in for an interview, but ultimately released each without charges, a level of caution that one police source said was indicative of the pressure detectives are under to get the case right.

“They’re afraid of comparisons to the Central Park Five, so they want to make sure they have as much evidence as possible, this way they’re not second-guessed,” said the insider.

In assembling that evidence, cops executed search warrants at the homes of each of the teens, whose names are being withheld by The Post because they are yet to be charged with a crime.

The tour through the home of the suspected stabber proved particularly fruitful, with investigators finding an empty plastic baggie in the style of those commonly used by drug dealers, according to sources.

Cops showed that baggie to a local pot peddler, who positively identified it as one he’d sold to Majors shortly before her murder, sources said.

The case was stalled, however, when that teen went off the radar for more than two weeks, sources said.

After cops found him in the Bronx, he was briefly detained for questioning and a cheek swab, but cut loose, sources said.

Investigators hoped to match the DNA from that sample to genetic material found both at the scene and in Majors’ teeth, after she clamped down on the alleged stabber — but sources now say that those results didn’t pan out to be quite so convincing.

It remains unclear what hard evidence, if any, has been culled from a knife found in the park, which police sources first indicated last week is the murder weapon.

Authorities aren’t taking any chances, preparing a massive presentation of evidence to take to a grand jury, sources have said.

Last week, dozens of court notifications and subpoenas went out, both to a wide array of NYPD personnel — ranging from responding officers to crime-scene and lab detectives — and civilians, including at least one witness and the drug dealer who allegedly sold to Tessa, sources said.

With or without a conclusive DNA match, authorities believe they can move the case forward, according to one source.

“They feel they have enough evidence to get an indictment,” said the insider.

Additional reporting by Craig McCarthy