Lashanda Armstrong had 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, the only survivor as his mother and three siblings drowned, says she had told the children, "You’re all going to die with me," but then tried, too late, to back out of the river, according to a woman who found the sopping wet boy.

Meave Ryan was driving past the boat ramp in Newburgh, about 60 miles north of New York City, on Tuesday evening when she spotted the boy, La’Shaun Armstrong, 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 La’Shaun said Armstrong had held him and the other kids — boys ages 5 and 2 and an 11-month-old girl — 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,’" Ryan said. "She said that two or three times."

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 drove the boy to a nearby fire station. Rescuers immediately went to the river, but it was too late: They found the van about 25 yards from shore in 8 feet of water. Lashanda Armstrong and three children were dead.

Ryan’s account conforms to reports from police, who say 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," Newburgh police Chief Michael Ferrara said.

Lt. Bruce Campbell said police had heard from at least two witnesses who said they saw a minivan speed through an intersection at the boat launch’s entrance fast enough that the vehicle appeared to bottom out on the bumpy pavement.

"She went through it at a pretty good pace," Campbell said.

The witnesses, motorists who had stopped at a stop sign, "just thought it was kids driving fast or acting up," he said. They apparently didn’t see what happened next, he said.

Campbell said police believe Armstrong hit the water going 15 to 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, laid a red rose on the doorway to Armstrong’s apartment Thursday morning. The two women were in the same math class at Orange County Community College. George, of Newburgh, 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."

People lit candles near the spot where the van entered the water. Natasha Colon and Nicole Callahan, both mothers from Newburgh, were among the mourners.

"I just wanted to say a prayer for them and for the boy who’s going to go through a lot," Colon said.

Callahan added: "They were innocent babies."

Jay Vandervort, a Newburgh resident who said he was a friend of Armstrong’s, also stopped by the boat ramp. He said he last saw her about a month ago.

"She just seemed happy-go-lucky, like everything’s good," he said.

A few dozen people gathered on the landing Thursday evening next to the makeshift shrine to offer up prayers for the family and toss handfuls of symbolically cleansing salt into the river.

Armstrong’s aunt had called police at about 7:45 p.m. Tuesday to report a domestic incident at her niece’s apartment. By the time police got there, she was gone. Besides Armstrong, Landen Pierre, 5, Lance Pierre, 2, and 11-month-old Laianna Pierre died.

Earlier that day, Armstrong appeared stressed when she picked up the children at the Young and Unique Christian Development Child Care, said Shaniesha Strange, supervisor in the infant room.

Police questioned the man they identified as the father of the three dead children, Jean Pierre, but did not give details. He apparently didn’t live with the mother and children and could not immediately be located for comment.

Police on Thursday confirmed that Pierre was charged with child endangerment in February when a boy who died in the minivan was found wandering the streets half-naked after 1 a.m. No further details were released by police, and the status of the case was not immediately available.

Hetty Minatee, another teacher at the day care center, said Armstrong had enrolled the four children there in September. At first, Jean Pierre would come in with Armstrong and sometimes would pick up the kids.

"A couple weeks ago, she came in a little upset," Minatee said. "She said, ‘Miss Minatee, I don’t want the father to pick the kids up or have any contact with them.’ She said she was trying to get a court order so he could never see the kids again."

La’Shaun is staying with the aunt, Angela Gilliam. He’s "doing fine," Gilliam said.

"She was a good mother. She was going through some stuff," Gilliam said of her niece. "Nobody knows what my niece went through."

She would not elaborate.

The boy told Ryan that Armstrong made a cellphone call to her mother as she sped toward the water, asking for forgiveness.

Ryan said La’Shaun blames himself for never teaching his siblings how to swim.

Ryan said she kissed him on the head before she left and told him, "You’re in my prayers."

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Associated Press writers Mary Esch and Chris Carola in Albany, N.Y., contributed to this report.