At nearly every step since the Deepwater Horizon exploded more than a month ago, causing the worst oil spill in U.S. history, rig operator BP PLC has downplayed the severity of the catastrophe in the Gulf of Mexico.

On almost every issue — the amount of gushing oil, the environmental impact, even how to stop the leak — BP’s statements have proven wrong. The erosion of the company’s credibility may prove as difficult to stop as the oil spewing from the sea floor.

Workers clean up oil residue Saturday along the beach in Port Fourchon, La.

“They keep making one mistake after another. That gives the impression that they’re hiding things,” said U.S. Sen. Bill Nelson, a Florida Democrat who has been critical of BP’s reluctance to publicly release videos of the underwater gusher. “These guys either do not have any sense of accountability to the public or they are Neanderthals when it comes to public relations.”

Take one of the most obvious questions since the April 20 explosion: How much oil is leaking? Official estimates have grown steadily — first the word was none, then it was 42,000 gallons, then 210,000 gallons. And now a team of scientists say the leak may well be five times that, making the spill worse than the Exxon Valdez.

All the while, BP has been slow to acknowledge the leak was likely much worse than the public had been told.

The oil giant’s behavior has led to accusations that it has been motivated to keep the leak estimate low because under federal law the size of eventual fines is tied to the size of the leak.

Underestimating the oil

Nelson said that he believes BP has delayed release of everything from the actual flow rate to the videos because of a federal law that allows the government to seek penalties of $1,000 to $4,300 per barrel — 42 gallons — of oil spilled in U.S. waters. “And so naturally they want to minimize what people were thinking they were going to spill.”

High-end estimates by BP, the Coast Guard and the National Oceanic and Atmospheric Administration reached 588,000 gallons per day in late April, BP spokesman David Nicholas acknowledged Friday to The Associated Press after weeks of the company sticking with the lower estimate. But it wasn’t until Thursday that officials had conceded that the leak was considerably larger than the 210,000-gallon-a-day figure that had been floated as the best estimate for the prior four weeks.

Even before the accident, there were indications that BP could vastly underestimate an oil spill’s likely size. In its regional spill response plan for the Gulf, the formula BP proposed to use to estimate the volume of oil in a surface sheen was smaller by a factor of 100 from the accepted international standard, which is also the basis for estimates by NOAA, the federal agency tasked with such calculations.

Nicholas said he doesn’t know where the numbers in BP’s plan “were derived, but they were not used” in calculating the amount of oil that had reached the Gulf’s surface since the accident. He also emphasized that the official estimates were not BP’s alone, but rather a collaboration with government agencies.

With criticism continuing to mount, when he was pressed Friday about BP’s perceived lack of transparency, Chief Operating Officer Doug Suttles said: “We’re trying to provide as much data as we can. We’re in the middle of this operation. … There’s a tremendous amount of transparency here.”

Nicholas said only Friday that daily estimates from April 27 through April 30 were based on two scientific standards. The “low end” was always around 42,000 gallons per day, the “best guess” was between 210,000-252,000 gallons per day, and the “high end” varied from 504,000-to-588,000 gallons per day, he said.

The 210,000-gallon estimate that became the official talking point for weeks turned out to be wrong, too. A team of scientists from the government and academia said Thursday that the leak is really spewing somewhere between 500,000 and a million gallons a day.

The new estimates were between 12 and 24 times greater than what was first offered, and instantly made the Deepwater Horizon spill the worst in U.S. history. Even using the low end of the estimates, nearly 18 million gallons have spilled so far. At the high end, the well could have gushed as many as 39 million gallons.

Obama frustrated

Even President Barack Obama has voiced his frustration, laying the blame squarely on BP for the often incorrect assessment of the spill’s size.

“Their interest may be to minimize the damage and, to the extent that they have better information than anybody else, to not be fully forthcoming,” Obama told reporters Thursday. “So my attitude is, we have to verify whatever it is they say about the damage.”

To be sure, experts say there’s no easy way to measure a leak 5,000 feet deep. Some estimates were based on satellite images or flyovers. The federal government has worked closely with BP, and Obama has acknowledged shortcomings, but it’s BP that controls much of the technology, like underwater robots that capture video of the leak.

Obama noted that BP kept video of the leak and didn’t make it public.

BP’s Nicholas said the government “has had access to the video since the incident started.”

Perhaps if BP, one of the wealthiest companies in the world, had released the video to the public and independent experts, it would have led to more accurate assessments of the spill’s size early on.