NEWBURGH, N.Y. – The suicidal mother who loaded her four children into a minivan and drove it into the frigid Hudson River warned in a cryptic Facebook message just before the fatal plunge: “I’m so sorry everyone forgive me please for what I’m gonna do…. This Is It!!!!”

Her 10-year-old son, who swam ashore as his mom and three siblings drowned, says his mother told her children, “you’re all going to die with me,” then changed her mind and tried, too late, to back out of the river, according to the woman who found the sopping wet boy.

“I made a mistake, I made a terrible mistake,” the boy quoted his mother as saying.

Just before 8 p.m., Lashanda Armstrong put her children into the van and drove into the river. Armstrong and three of the children, boys ages 5 and 2 and an 11-month-old girl, died. Ten-year-old La’Shaun Armstrong escaped.

Meave Ryan was driving past the boat ramp in Newburgh on Tuesday evening when she spotted La’Shaun waving his arms.

Ryan said the boy told her that his mother had had a “big, big argument about my stepdad’s cheating on her,” then piled the four children in the minivan and sped into the river.

Ryan told The Associated Press on Thursday that the boy said Armstrong had held the kids as the minivan began sinking.

“She was holding on to all of them and said, ‘If I’m going to die, you’re all going to die with me.’ She said that two or three times,” Ryan said.

But as the vehicle sank, La’Shaun broke free of his mother’s grasp and clambered out a window.

Ryan said: “While he was doing that, he heard his mother saying, ‘I made a mistake, I made a terrible mistake.’ And she tried to reverse the car out, but at that time it was too late. He said, ‘Mommy, I’m going to go get help,’ and she said, ‘OK.’ And that was the last he heard from his mother.”

Ryan’s account conforms to reports from police, who say Lashanda Armstrong was involved in a domestic incident at her apartment Tuesday evening and within minutes had plunged off a boat ramp into the river just a half-mile from her apartment in a hard-luck section of the city.

The Facebook message, posted from Armstrong’s BlackBerry, was obtained by MidHudsonNews.com.

It posted on her page at 7:13 p.m. Tuesday, about a half-hour before she drove into the river.

“I don’t know if we’ll ever find out what Lashanda Armstrong was thinking when she left that home and drove to the river,” said Newburgh Police Chief Michael Ferrara.

Campbell said police believe she hit the water going 15-20 mph.

There was no substantial damage to the vehicle’s front end, and the airbags didn’t deploy, he said.

A stream of people went to the boat ramp Thursday and friends and neighbors gathered to try to process the tragedy.

Ashonti George, 21, of Newburgh laid a single red rose on the doorway to Armstrong’s apartment on Thursday morning.

The two women were in the same math class at Orange County Community College. George said that Armstrong usually was an attentive student but wasn’t herself during a Tuesday morning test.

“She was off,” George said. “She didn’t seem like herself. She seemed angry — off. That’s the best way I can explain it.”