The feds want to know why Assemblyman and Brooklyn Democratic boss Vito Lopez doesn't like tall buildings.

Sources say federal prosecutors are investigating whether a Lopez-backed proposal for new low-income, low-rise apartments was a political favor to Hasidic Jews, who prefer living close to ground level because they don't ride elevators on the Sabbath.

The Brooklyn US Attorney's Office is weighing whether to make a case that the project is discriminatory, intended for religious Jews while leaving blacks and Hispanics out in the cold.

Prosecutor Michael Goldberger, who heads the office's civil-rights bureau, recently inspected the site, a swath of mostly vacant and industrial lots on the Bedford-Stuyvesant/Williamsburg border called the Broadway Triangle, sources said.

The feds' interest comes after a state judge threatened to put the $50 million, 159-unit housing proposal on ice over questions as to whether the city's decision to build Hasidic-friendly housing was "preordained," despite the city's claim that it considered proposals for taller buildings.

Lopez's nonprofit housing empire, the Ridgewood Bushwick Senior Citizens Council, partnered with United Jewish Organizations to pitch a housing deal for the Broadway Triangle, city land that Williamsburg's Hasidim had long coveted.

The two politically connected nonprofits asked for more than $38 million in taxpayer funds to build two seven-story buildings.

Without giving other groups a chance to apply, the city quietly offered the partnership public land valued at $19 million for a mere $13 -- a buck for each vacant lot -- if they could secure state funding. The city Planning Commission also rezoned the area so no buildings could exceed eight stories.

"We're not opposed to the first eight floors' being configured to accommodate the needs of Hasidic families, but why can't we build up to 20 stories and accommodate others?" said lawyer Martin Needelman, of Brooklyn Legal Services, which has sued in state court to block the project.

The Planning Commission ruled that slightly taller buildings would "far exceed the existing neighborhood context and would be inappropriate for this area" -- despite the fact that 22-story residential high-rises already stand four blocks away.

Angela Battaglia, Lopez's girlfriend and the housing director for Ridgewood Bushwick, sits on the commission. She recused herself from the vote.

The judge reviewing the project disagreed with the commission, saying taller buildings "would not insult the architectural integrity of the neighborhood." Judge Emily Jane Goodman in May further said it was "vexing" that the city would not want larger buildings given the "critical shortage of housing."

The city denied the plan was influenced by favoritism.

US attorney spokesman Robert Nardoza declined to confirm or deny any federal investigation.

Additional reporting by Brad Hamilton