Maria Acosta ran to her neighbor's house in St. Paul about 4:30 a.m. Thursday as flames engulfed a staircase that was the only exit for the second floor of a duplex.

"All of a sudden, it went, 'Whoosh!,'" Acosta said, recalling how fast the fire spread at the back of the building.

The first-floor tenant, Laura Haslach, escaped after being awakened by pounding on her back door. But she was horrified -- her daughter, Allison Haslach, and her daughter's fiancé and two young children were trapped upstairs.

"All I could think was, 'Please, don't let them die in a fire!'" Haslach said.

She reached for a hose, but it wasn't working. She threw a flowerpot at the couple's bedroom window, which was at the front of the house. Acosta grabbed flat, palm-sized rocks from her garden and pelted the window until the couple woke up about five minutes later.

Allison's fiancé smashed the window open. By then, other neighbors had gathered.

One neighbor called for a sheet to catch the tenants.

"No, no, no!" Acosta said. "Get a mattress!"

Acosta's husband, Walter, ran back to their home and dragged out a twin mattress with the help of their 19-year-old son.

They placed it under the window, and the children, 7-year-old Alexana and 4-year-old Thomas, were lowered into the arms of waiting neighbors.

Firefighters arrived and rescued Allison, 27, and her fiancé. Allison hurt her ankle coming down the ladder, and a firefighter was slightly hurt in a fall from the roof, said St. Paul Fire Marshal Steve Zaccard.

Authorities said they believe the fire, in the 1000 block of Marion Street, was arson.

"When someone intentionally sets fire to an occupied residence like this, blocking their escape, they're trying to kill someone," Zaccard said.

He declined to say how authorities determined that the fire was arson "because we are still looking for suspects."

Haslach said she has no idea what could have led anyone to set the duplex on fire and recalled nothing unusual in recent days.

Both levels of the duplex are uninhabitable, the fire marshal said.

Zaccard said the upstairs tenants "lost virtually everything" in the blaze, which was put out in about 20 minutes and caused about $120,000 in damage.

Haslach said she has renter's insurance but that her daughter doesn't.

"Whoever did it knew there were kids living there," Acosta said, adding that there were bikes, toys and a swing set in the backyard. "You know they had to have seen it."

The Red Cross is helping residents with housing and replacing belongings.

"My neighbors are heroes," Haslach said. • 612-673-4482 • 612-270-4708 Twitter: s@ChaoStrib