The California case, a rare test of the national security letter statute, involves an unnamed phone company that has been fighting the letter since 2011.

WSJ: Judge Strikes Down Secretive Surveillance Law

And school-district officials in Oklahoma complained recently about how test scores are factored into controversial new letter grades assigned to schools.

WSJ: Seattle Teachers Protest Exams

Halliburton then conducted another test using a modified procedure, the letter said, and the result for the first time indicated the slurry design would be stable.

CNN: STORY HIGHLIGHTS

The Republican letter released today does not meet the test of balance.

WHITEHOUSE: The White House

After a letter from the BBC in November, the test was finally carried out in April this year.

BBC: NEWS | Health | 'Please don't take my baby'

However, the letter said investigators were unsure if that fourth test was completed before the cement slurry was poured at the Macondo well, and that Halliburton reported the positive result to BP after the blowout.

CNN: STORY HIGHLIGHTS

Shortly after registering with a GP, he received a letter inviting him to go to the surgery for a smear test.

BBC: NEWS | UK | Magazine | Leslie, Kerry, Kim, Vivian, Tracy write