Mr. Rampe's departure comes at a particularly sensitive moment for a rebuilding effort that has recently been plagued by bad news. The construction of the Freedom Tower, the site's most prominent structure, is months behind schedule, and faces even greater delays after the New York Police Department last month delivered a disturbing assessment of its security shortcomings. Larry Silverstein, the lead developer of the site, has signaled that he may need hundreds of millions of dollars from the government to address the security concerns.

The decision by the Wall Street firm of Goldman Sachs to shelve plans for building a headquarters near ground zero was another setback. And to this day, three and a half years after the destruction of the two towers, a number of the architects, engineers and designers involved in the rebuilding effort have complained privately that a lack of creativity and communication still besets the project.