WASHINGTON — Since the Deepwater Horizon oil drilling rig exploded on April 20, the Obama administration has granted oil and gas companies at least 27 exemptions from doing in-depth environmental studies of oil exploration and production in the Gulf of Mexico.

The waivers were granted despite President Barack Obama’s vow that his administration would launch a “relentless response effort” to stop the leak and prevent more damage to the gulf. One of them was dated Friday — the day after Interior Secretary Ken Salazar said he was temporarily halting offshore drilling

The exemptions, known as “categorical exclusions,” were granted by the Interior Department’s Minerals Management Service (MMS) and included waiving detailed environmental studies for a BP exploration plan to be conducted at a depth of more than 4,000 feet and an Anadarko Petroleum Corp. exploration plan at more 9,000 feet.

“Is there a moratorium on off shore drilling or not?” asked Peter Galvin, conservation director with the Center for Biological Diversity, the environmental group that discovered the administration’s continued approval of the exemptions. “Possibly the worst environmental disaster in U.S. history has occurred and nothing appears to have changed.”"