LOS ANGELES – The federal agency responsible for ensuring that an oil rig in the Gulf of Mexico was operating safely before it exploded last month fell well short of its own policy that the oil rig be inspected at least once per month, an Associated Press investigation shows.

Since January 2005, the federal Minerals Management Service conducted at least 16 fewer inspections aboard the Deepwater Horizon than it should have under the policy, a dramatic fall from the frequency of prior years, according to the agency’s records.

Under a revised statement given to the AP on Sunday, MMS officials said the last infraction aboard the rig, which blew up April 20, killing 11 and spewing millions of gallons of oil into the Gulf of Mexico, occurred in August 2003, not March 2007 as originally stated.

The inspection gaps and poor recordkeeping are the latest in a series of questions raised about the agency’s oversight of the offshore oil drilling industry. Members of Congress and President Obama have criticized what they call the cozy relationship between regulators and oil companies and have vowed to reform MMS, which both regulates the industry and collects billions in royalties from it.

Earlier AP investigations have shown that the doomed rig was allowed to operate without safety documentation required by MMS regulations for the exact disaster scenario that occurred; that the cutoff valve that failed has repeatedly broken down at other wells in the years since regulators weakened testing requirements; and that regulation is so lax that some key safety aspects on rigs are decided almost entirely by the companies doing the work.

The AP sought to find out how many times government safety inspectors visited the Deepwater Horizon, and what they found. In response, MMS officials offered a changing series of numbers.

At first, officials said 83 inspections had been performed since the rig arrived in the Gulf 104 months ago, in September 2001. While being questioned about the once-per-month claim, the officials subsequently revised the total up to 88 inspections. The number of more recent inspections also changed — from 26 to 48 in the 64 months since January 2005.

No explanation was given initially for the upward revisions.