Permitted overflight of Washington DC

[quorm]

I notice that there are regular overflights of Washington DC airspace by commercial aircraft heading from NE to SW, usually between 26000 and 32000. These are not flights heading to DCA. They do not overfly forbidden areas such as the Mall and neighboring Federal areas.

I was under the impression that overflight of Washington DC is prohibited, but that's probably my misreading of FAA regs. Is there a permitting process that allows these flights? A link to the official regs and process would be helpful.

Thanks