Pennsylvania police on Monday arrested the mother of two young children who were found hanging 10 weeks ago, and charged her with their "horrific, tragic" murders, authorities said.

Lisa Rachelle Snyder, 36, was booked on suspicion of murder, endangering welfare of children, tampering with evidence and sexual intercourse with an animal, police and Berks County prosecutors said.

She's being held without bail.

Lisa Snyder was charged with the murder of her two children after they were found hanging in their Pennsylvania home in September. WCAU / WGAL

"This is a horrific, tragic incident," Berks County District Attorney John Adams told reporters. "We are very saddened by this incident that took place."

The children — 8-year-old Conner Snyder and 4-year-old Brinley Snyder — were found unresponsive and hanging in the basement of their Albany Township home, about 75 miles northwest of downtown Philadelphia, on Sept. 23, officials said.

Snyder had told police that Conner was suicidal because he had been bullied at school. But teachers and classmates of the youngster told the investigators that Conner was never bullied and didn't appear to be depressed, police said.

Adams said the defendant's story never added up.

"It would be safe to say that we had immediate questions," the prosecutor said. "Eight-year-olds, generally, that I am aware of, do not commit suicide, so of course we had questions."

Investigators are still trying to determine a motive for the murders.

“She had expressed to a friend that … she had had enough," Adams said. "I don’t know if there’s any explanation for her behavior at all. I don’t think I can stand up here … and explain the horrific loss of two innocent children's lives."

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The ensuing investigation also uncovered evidence of animal sex, according to Adams.

"During our search of the (suspect's) Facebook Messenger, we were able uncover information and photos with her having sex with her dog," Adams told NBC News earlier on Monday.

Snyder's attorney did not immediately return messages seeking his comment on Monday.

The victims were found hanging from a support beam and two dining room chairs were found knocked over near the children, officials said.

The children were hanging by a vinyl dog lead that Snyder had purchased online on Sept. 22 and picked up at a nearby Walmart on Sept. 23, according to Adams.

Conner struggled with dexterity and was working with a occupational therapist, provided by the local school district, officials said.

That therapist told police there's no way Conner, who couldn't tie his own shoelaces, would have been able to tie the nooses found around his and his sister's necks, according to Adams.

And in the days leading up Sept. 23, Snyder did Google searches for "carbon monoxide in a car, how long to die," "hanging yourself, "short drop, simple suspension" and "does a hybrid car produce carbon monoxide."

"The autopsies ... determined the cause of death for both children was hanging," Adams said. "The Lehigh County coroner has ruled these deaths as homicide."

A 17-year-old boy also lived in the house but he was not involved with the crime, Adams added. The father of the children did not live with the family.

The teenager is now staying with other family and the victimized dog had already been given away to a Facebook friend of the suspect's before Monday's arrest, Pennsylvania State Police Trooper David Beohm said.