Members of a nonprofit founded in memory of a firefighter killed in Sept. 11 hope to raise $800,000 by the end of the year to pay off the mortgages of slain New York City police officers Wenjian Liu and Rafael Ramos.

The Stephen Siller Towers to Tunnel Foundation said it has already raised $70,000, including a $20,000 pledge from former New York Mayor Rudy Giuliani, chair of the foundation's annual run for years.

"I think it's important that we show the Liu and the Ramos family that we are all coming together as a community, as New Yorkers and as Americans," chair and CEO Frank Siller, whose brother Stephen died responding to the attacks, told reporters Friday at the foundation's Staten Island headquarters.

Ramos, 40, and Liu, 32, were killed in an ambush attack while sitting in their squad car in the Bedford-Stuyvesant section of Brooklyn on Dec. 20.

The foundation -- named in honor of Stephen Siller, who grew up in Rockville Centre -- learned of the Liu family's financial difficulties through press reports and statements by Cuomo, Frank Siller said.

Liu's wife, Pei Xia Chen, held her head down and sobbed during the news conference. Her family members didn't speak but they released a statement thanking the nonprofit, Giuliani and Cuomo and "everyone who has shown their support at this difficult time."

In a statement, Cuomo commended the foundation and its important work to "honor their memory by doing all we can . . . to take care of their families now and in years to come."

The foundation builds houses nationwide for catastrophically injured service members returning from Iraq and Afghanistan.

Giuliani, who attended the news conference, urged others to donate.

"It doesn't bring back either one of these great men, but it does help," said Giuliani, who said he visited the families of both officers on Tuesday. He said the officers "died for us . . . so that last night, at Christmas, we could go to bed with our children safe."

The PBA and city have benefits for families of slain officers, Giuliani said, but he doesn't believe the Liu family knows about them yet because of a language barrier and "there is also an emotional thing they're going through."

Giuliani said Liu, a seven-year veteran, was an only child and the family breadwinner.

Ramos was married and had two sons, Giuliani said, and the officer worked overtime to pay his eldest son's tuition at Bowdoin College.

Ramos lived in Cypress Hills, Brooklyn. Liu lived in Brooklyn's Gravesend neighborhood.

"We don't get officers like Officer Liu and Officer Ramos from thin air . . . they're brought up right," Giuliani said. "We owe these families."