BANGOR – As jurors were deliberating the fate of accused murderer Jeffery Cookson last December, a Maine state firearms examiner who provided crucial testimony that helped convict Cookson was realizing he had made a big mistake.

On Thursday, Bryan Bachelder, a firearms examiner for the Maine State Police Crime Lab, testified about the mistake during a hearing at Penobscot County Superior Court on Cookson’s motion for a new trial.

The mistake, coupled with the confession of another man to the 1999 double shooting deaths of 21-month-old Treven Cunningham and 20-year-old Mindy Gould in Dexter, formed the basis for Cookson’s motion.

During Cookson’s trial, the state had no murder weapon to show jurors. It did, however, have spent bullets and shell casings from the murder scene and knew they had been fired from a Taurus 9 mm handgun. Investigators had learned that Cookson owned a Taurus and had traded a generator for it in 1997 with a man in Brunswick. With that, investigators spent weeks tracking down previous owners of the gun, which had been traded or sold several times. Many of the previous owners turned over spent casings or bullets that had been fired from that gun.

Bachelder conducted tests on all of the casings and bullets collected from the previous owners, Cookson’s front lawn, and the murder scene, and determined they had all be fired from the same gun.

The information provided prosecutors and jurors with a direct link between Cookson and the murder scene.

But Bachelder said Thursday that the shell casings did not match and in fact were not fired from the same gun. Bachelder said a marking on each casing and bullet was new to him and, he thought, unique to one gun. What he learned, while reading a professional publication months after making the finding, was that the marking was common to all Taurus or Beretta firearms.

So the gun that Cookson acquired from the Brunswick man was not the murder weapon, as the prosecution had claimed.

Bachelder reportedly rushed to notify prosecutors, but the guilty verdict had already been delivered.

Bachelder’s testimony was the latest surprising revelation in a case that has been marked by almost unbelievable twists and turns since the jury announced its guilty verdict on Dec. 3.

Ironically, Bachelder’s bombshell was not the reason for Cookson’s motion for a new trial. Only minutes after the verdict, it was revealed that another man had confessed to the killings. That man, David Vantol, also turned over the elusive – and actual – murder weapon to police.

Cookson’s attorney, William Maselli of Auburn, delivered that news to the judge and prosecutors moments after the jury left the courthouse. He then filed a motion for a new trial.

It wasn’t until the next day that prosecutors learned of Bachelder’s error. At the same time, state police investigators were rushing to find out if Vantol’s confession was legitimate.

Hearings on Cookson’s motion began earlier this month and had been continued until Thursday.

At the first hearing, Vantol recanted his confession on the witness stand and said Cookson had summoned him to the Penobscot County Jail during the second week of the trial and persuaded him to confess to the murders. Records show that a day after Vantol visited the jail, he made the confession to Cookson’s attorney. Vantol was friends with Cookson and at the time lived with Cookson’s brother, Scott Cookson, in Guilford.

Staff from Acadia Hospital, where Vantol has been treated, testified Thursday that Vantol has a lower than average IQ, is easily influenced by other people and suffers from severe depression, psychosis and fetal alcohol syndrome.

They further testified that they had difficulty believing that Vantol had committed the murders.

Despite the confession that came at the time of the trial, Maselli chose not to put Vantol on the stand because the young man planned to testify that although he pulled the trigger, Cookson had put him up to it. Under Maine law, that would still make Cookson guilty of murder. Instead, Maselli waited for the verdict, which was guilty, and with nothing to lose came forward with the Vantol information.

On Thursday, Bachelder testified that the shell casings from the murder scene and the spent bullet taken from Cookson’s front yard were both fired from the gun that Vantol turned over to police. Vantol has testified that Cookson told him where to find the gun.

Now prosecutors believe that their case against Cookson is stronger than ever. Now they have the real murder weapon and though they have no way to trace ownership of the weapon to Cookson, they do have evidence that the spent bullet from Cookson’s property was fired from that gun and so were the bullets that killed Gould and Cunningham.

“We know that the gun that killed Mindy and Treven was once at Cookson’s house,” said Assistant Attorney General Lisa Marchese.

Meanwhile, officials at the Maine State Police Crime Lab are working on making sure that their mistake is not repeated.

“We are reviewing all [firearm] identifications made by [Bachelder] and Chuck Hjelm, [another examiner who agreed with Bachelder’s original finding] to make sure they are correct,” said Maine State Crime Lab Director Tim Kupferschmidt. “We are being more cautious and will undergo additional and intensive training in this area.”

Whether Cookson gets a new trial will be decided by Justice Roland Cole, who will use trial transcripts, testimony from this week’s hearing, and oral and written arguments from attorneys to sort out the whole mess.

Oral arguments from the attorneys are expected to occur in May, and then Cole will begin the decision-making process and issue a written decision.

Cole said Thursday that he expected that no matter what his decision is, it will be appealed by one side or the other.

Meanwhile, Cookson remains in jail.