Two weeks ago the House of Representatives voted to bar the Obama administration from engaging in a controversial surveillance practice that insiders call a "backdoor search." A Friday letter from the Obama administration gives some hints about how common the practice is.

The letter, addressed to Sen. Ron Wyden (D-OR), admits that the National Security Agency, the Central Intelligence Agency, and the Federal Bureau of Investigation all conducted backdoor searches in 2013. Thousands of Americans were search subjects. Most of these were searches of metadata, such as what number people called and when. But at least 198 searches — and possibly many more — were seeking the contents of Americans' private communications.

The 2008 FISA Amendments Act gave the NSA authority to engage in warrantless surveillance of communications between Americans and foreigners. But the law included an important caveat: an American could not be the "target" of warrantless spying. Americans' communications were only supposed to be collected incidentally in the process of spying on foreigners.

However, the law allows the government to pool all the messages it intercepts into a giant database — and then search the database for the communications of Americans. This is known as a "backdoor search."

Sen. Ron Wyden, a longtime NSA critic, has been pressing the Obama administration to provide more details on this aspect of its operations. The Obama administration complied last week, admitting it had conducted such searches thousands of times in 2013.

Here's how often government agencies engaged in the controversial practice last year:

The NSA brass approved searches for the contents of the communications of 198 Americans. Such content searches could include the audio of phone calls or the body of emails. In some cases, NSA analysts conducted multiple searches for the same person.

The NSA conducted approximately 9,500 searches for Americans' metadata — information such as phone numbers dialed, email recipients, and the length of calls. About a third of these were duplicate queries for the same individual.

The CIA conducted "fewer than 1,900 queries" for information about Americans, of which 27 percent were duplicate searches for the same American. The letter doesn't say these are content queries, but a later statement that "CIA does not track the number of metadata-only queries" suggests that they were content queries.

The letter says the FBI doesn't keep track of how many queries it has performed, but "the FBI believes the number of queries is substantial." On the other hand, the FBI "only requests and receives a small percentage" of the NSA's collection of data acquired under the FISA Amendments Act.

This last item is significant because it suggests that information ostensibly collected for foreign intelligence purposes is routinely being used for domestic law enforcement. We don't know how often this occurs, or what safeguards are in place to protect Americans' privacy.

The Obama administration argues that the process is totally legal, and that the term "backdoor search" is a misnomer. The government insists that the searches are "lawful, limited in scope, and subject to oversight as approved by the Foreign Intelligence Court (FISC)." It also insists that its surveillance is "not bulk collection."

Yet a new report from the Washington Post suggests how vast the NSA's dragnet could be. The Post reports that the FISC has approved surveillance of 193 foreign governments. These authorizations not only allow interception of these governments' communications, it also includes communications about these governments, even if the governments themselves are not party to the communications.

A dragnet that large could sweep in a lot of Americans' private communications that have no plausible connection to terrorism or spycraft. And "backdoor searches" could allow US agencies to snoop through it.