Alleged copkiller Eric Frein knew of one of the state troopers who was shot in a shift-change ambush — and he may have believed the trooper was having an affair with his own sister-in-law, MailOnline can reveal exclusively.

Alex Douglass, who was critically injured after being hit in the pelvis, lives less than half a mile from the family of Frein's brother and was friends with the fugitive's sister-in-law Melissa.

Police say they have ruled out the possibility of an affair after 'extensive interviews' with Mrs. Frein, the wife of Michael Frein, which are understood to have included a polygraph test.

'We have investigated the rumor of the affair thoroughly after we learned about it', Pennsylvania Sate police spokesman Trooper Tom Kelly told MailOnline. 'There is no truth to the affair.

Scroll down for video

Rambo: Roman Kamensky took this picture of Eric Frein in April 2009 in Canadensis, Pennsylvania. Frein was looking out of his back window, wearing a Serbian uniform from the 1990s at the time of the break-up of the former Yugoslavia. 'It was what is referred to as an "impression" picture,' said Kamensky. 'We take them to create a historical image'

Hunt: Police and U.S. Marshals clear homes in the search for Eric Frein in Canadensis on Wednesday

'We used thorough investigative techniques and questioned Melissa Frein extensively. She denied it and we do not believe she is being deceptive.'

Kelly confirmed that Douglass and Melissa Frein are friends and there is no way to know whether Eric Frein believed that his sister-in-law was involved with the 31-year-old trooper until and unless they manage to arrest him.

Frein was the best man for his brother Michael when he married Melissa in 2007.

Melissa Frein did not return calls from MailOnline.

MailOnline also learned that she has recently left her place of employment, where she was an accountant.

Her husband, Michael, said: 'I have no comment. Talk to the police' before shutting the door of his home in Olyphant, Pennsylvania.

Outside the new 4-bedroom, 2-bath house that the Freins bought in December 2007, the mailbox is festooned with a big blue ribbon, a sign of support for police as they search for Eric Frein.

One neighbor told MailOnline he hasn't seen Melissa at the home in more than a week, although he normally saw her every day. However Kelly said: 'As far as we know they are still together, living in the same residence.'

The neighbor revealed SWAT teams descended on Michael Frein's home after the shootings and took him away in handcuffs. He was later released and police say his brother is the only suspect in the case.

Target? The story that state trooper Alex Douglass, who survived the shooting, and Melissa Frein, Eric Frein's sister-in-law, were having an affair has spread like wildfire through the towns surrounding Canadensis

Douglass is still recovering in hospital, 13 days after he was shot at the state police barracks at Blooming Grove, Pennsylvania as he turned up for work. Another trooper, father-of-two Bryon Dickson was killed.

Since then, hundreds of police from all over Pennsylvania have been joined by officers from neighboring New York and New Jersey as well as members of the FBI, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and federal marshals as they search for Frein in woods south of the picturesque Poconos town of Canadensis.

Local residents have been turned out of their houses, searched and in some cases had to spend nights in their cars as police carry out their search.

Frein, a survivalist, is said by police to harbor a deep grudge against law enforcement. They say they do not believe he will target members of the public but they think he plans to kill more officers.

If Frein believed his sister-in-law was having an affair with Douglass, it could help explain why he traveled 25 miles to Blooming Grove to carry out his deadly attack rather than pick a target closer to his home just south of Canadensis.

It was on his way back home that he plowed his green 2001 Jeep Suburban into a swamp after he failed to negotiate a junction. He is then believed to have made his way home through thick forest on foot.

The story that Douglass and Melissa Frein were having an affair has spread like wildfire through the towns surrounding Canadensis. One person told MailOnline that that is why Frein aimed at Douglass' groin area. Kelly said Douglass was shot 'in the pelvis, around the holster area.'

Lawyer Joshua Prince, who has been highly critical of the police action and is looking for clients who claim they have been illegally searched, evicted or refused entry to their homes, told MailOnline that if the affair had been true, state police would have been misleading the public into thinking that Frein was angry at all police rather than just one officer.

Taken: A neighbor said SWAT teams descended on the home of Michael Frein, Eric Frein's brother, after the shootings and took him away in handcuffs. He was later released.

Cleared: Michael Frein later released and police say his brother is the only suspect in the case

Where is Melissa? Outside the 4-bedroom, 2-bath house that the Freins bought in December 2007, the mailbox is festooned with a big blue ribbon, a sign of support for police as they search for Eric Frein. A neighbor tells MailOnline that Frein's s may have suspected was having an affair with state trooper Douglass, hasn't been seen in days

In a post on his website, Prince wrote: 'What if Eric Frein doesn’t have a vendetta against law enforcement? What if the shooting of the PSP Cpl. Bryon Dickson and Trooper Alex Douglas (sic) was in relation to a love triangle?

'If it is truly a personal vendetta against Trooper Douglas and not the Pennsylvania State Police or local law enforcement, the PSP/FBI should inform the public of this, so that the fear and panic that has been caused by their actions can be quelled.'

Prince told MailOnline he believes the state police have already convicted Frein, especially as Lt. Col. George Bivens confirmed on Wednesday that officers are authorized to use lethal force if Frein refuses to surrender.

'The PSP is acting as judge, jury and executioner,' said Prince. 'Bivens has activated a shoot-on-sight order unless he "actively surrenders." How does Eric Frein even know about that? If he turns and runs they are going to shoot him in the back."

According to the police complaint, Frein shot Dickson soon after the trooper turned up for his 3-11 pm shift at Blooming Grove. 'At approximately 2250 hours, Corporal Dickson was exiting the front door of the Blooming Grove station towards his vehicle which was parked in front of the station. As Corporal Dickson exited the front door, he immediately dropped to the ground in front of the door where he remained motionless,' the complaint reads.

Another trooper, Nicole Palmer, rushed to his aid and Dickson told her he had been shot and needed help. Police say Frein then shot again without hitting anyone. She tried to drag Dickson inside the building but could not.

Minutes later Douglass went to help his dying colleague. 'As Trooper Douglass approached Corporal Dickson, he looked into the lobby, at which time he fell to the ground,' reads the police complaint.

'Trooper Douglass was then able to crawl into the lobby' where he was rescued by another officer, Trooper William Fells.

Police say a total of four shots from a .308 rifle were fired, all within a 90-second period.

Method to his madness? Police admit that Frein may have believed that Douglass was having an affair with his sister- in-law. Meanwhile, police have discovered Serbian cigarette packets in the woods left by Frein, right

The hunt for Frein has dragged on for nearly two weeks. On Wednesday, Bivens revealed police have found soiled diapers and Serbian cigarettes in the dense forest area where they believe he is holed up. Frein is known to be obsessed with Serbia and has often played the role of soldiers from that country in military simulation exercises.

In an interview with MailOnline, Roman Kamensky, an acquaintance of Frein's who took the picture of him in April 2009 in Frein's home in Canadensis, Pennsylvania, says. 'We were all taken aback by this. It is very disturbing and way out of left field.'

He says Frein was looking out of his back window, wearing a Serbian uniform from the 1990s at the time of the break-up of the former Yugoslavia.

'It was what re-enacters refer to as an "impression" picture,' said Kamensky. 'We take them to create a historical image.'

Kamensky said Frein was not totally obsessed with Serbia as he was interested in other historical military periods including Germans in World War II, early Russian Federation and Americans in Vietnam.

'I knew him from the military simulation community. We were in contact over a period of two years. But it's been four or five years since I last had contact with him.'

Kamensky said the idea that his former friend might be a cop killer had come as a 'complete shock' not only to him but to other re-enactors.

'Of course I have known people who have had grudges against law enforcement - but he wasn't one of them.'

Bivens said survivalist texts found in Frein's bedroom advised him to wear diapers so he could stay in one place for lengthy periods.