Privacy advocates urge more transparency about how government keeps an eye on people

As Congress considers whether to extend a program that sweeps up American phone records, privacy advocates fear there are still too many unknowns about government surveillance to allow for a comprehensive public understanding.

The government has released reams of once-classified documents following the 2013 disclosures of the National Security Agency's surveillance efforts. But critics say key language remains censored and the release of information has been selective.

The NSA program that collects and stores phone records is conducted under Section 215 of the USA Patriot Act. Its vast reach remained secret until a former NSA systems administrator, Edward Snowden, disclosed details in 2013.

Congress is deciding whether to renew or modify the phone records collection when the law authorizing it expires in June. Legislation is expected to be unveiled Wednesday.