Mom Lynda Cruz paid $50,000 to adopt alleged school shooter Nikolas Cruz from his “drug addict” biological mother when he was just 3 days old, and then dropped another $15,000 on his younger half-brother, Zachary, when she found out the troubled mom was pregnant again, according to a report.

“Nik’s biological mother was just a complete screw-up, drug addict and thief,” former friend and neighbor Trish Duvaney told RadarOnline.

“I even said to Lynda: ‘You don’t know the mother’s background.’ Both children were born after one-night stands. The birth mother, she didn’t even really know who the two biological fathers were.”

Lynda was about 50 at the time, and she and husband Roger Cruz were struggling to adopt a child because of her relatively advanced age, Duvaney said.

“So she went to a lawyer and did it to where you pay the biological mother’s expenses,” she said. “You’re not directly giving money to the birth mother.”

Lynda ponied up $50,000 to get Nik, now 19, directly from the hospital when he was 3 days old — and another $15,000 to later adopt Zachary, now 18, Radar reported.

“She was happy with Nikolas and wasn’t going to try for another one, but when she found out he would have a biological brother, she decided right away to take him, and her husband was on board,” Duvaney said.

The biological mother was in prison when she gave birth to Zachary, according to Duvaney.

Duvaney, who lived next door to the family from 1999 to 2002, said Nik displayed signs he was deeply troubled even as a toddler and that “everybody dropped the ball on him.”

“He threw my 4-month-old into the pool. My son was crawling on the back patio and he threw my son into the pool. And Nikolas was only 2 then,” she said.

Nikolas watched Roger die of a heart condition in 2004 when the youngster was just 5. Lynda died of pneumonia Nov. 1 — less than three months before Nikolas allegedly opened fire on his former high school on Valentine’s Day, killing 14 students and three staffers.

Zachary was temporarily committed to a mental health facility following the shooting because “he was shocked. They (Zachary and Nikolas) were close growing up,” said Duvaney, who noted Zachary was a “laid-back and a loving little baby.”

A Facebook account believed to belong to Zachary recently began posting photos of the pair as children.