A former BP executive charged with obstructing a congressional investigation in the weeks after the 2010 Deepwater Horizon explosion is set to stand trial Monday in a case that features an unusual legal clash over whether a sitting U.S. senator and other high-ranking witnesses should be required to testify.

David Rainey, BP’s former vice president of exploration for the Gulf of Mexico, is the highest-ranking executive charged in connection with the disaster, which killed 11 men and led to the largest oil spill in U.S. history.

Rainey’s trial, expected to last three weeks, will bring a renewed focus on the federal government’s effort to hold individuals — and not just faceless corporate entities — criminally liable for the spill. Prosecutors allege Rainey deliberately withheld information in a May 2010 congressional briefing about how much oil was flowing from BP’s busted well and then made false statements to investigators about how he calculated the numbers.

BP stood by Rainey’s figures for weeks after the disaster despite mounting skepticism — even from within the oil giant — that they were far too low.

In 2013, U.S. District Judge Kurt Engelhardt threw out the obstruction count against Rainey, agreeing with a technical point made by the executive’s attorneys — that the congressional subcommittee that took his testimony wasn’t authorized to lead the investigation in the first place, and that a full committee was the proper authority. But the 5th U.S. Circuit Court of Appeals disagreed with Engelhardt and reinstated the charge.

Unlike the manslaughter charges prosecutors are pursuing against the two BP supervisors who were aboard the rig when it exploded, Rainey’s alleged misdeeds occurred after the deadly accident. The trial will focus on what he knew about the oil’s flow and whether he sought to skew potentially dire predictions during his subcommittee debriefing and in a 2011 interview with federal investigators.

In that way, the Rainey case is similar to the case against former BP engineer Kurt Mix, who is accused of obstructing justice by deleting text messages that dealt with the flow rate.

Last year, Mix, who was initially convicted, was granted a new trial by U.S. District Judge Stanwood Duval Jr., citing juror misconduct. The feds have appealed Duval’s ruling, and arguments are set to be heard this week by a three-judge panel of the 5th Circuit.

Geologist by trade

After BP’s Macondo well 50 miles off the Louisiana coast blew out on April 20, 2010, Rainey was the No. 2 official at BP’s Unified Command center in Robert, where cleanup and response efforts were coordinated.

A geologist by trade, Rainey had no experience determining flow rates, court records show, and he got involved in the work only after reading about how to do it online.

In notes from his FBI interview, Rainey comes off as a guy willing to give it a shot in a room full of reluctant engineers. “No one wanted to do the calculation,” the notes say. “Rainey is an ‘explorer.’ Explorers live in tension all the time. When trying to make predictions, explorers talk in ranges. At the end of the day they like ranges. Engineers like one number.”

In the weeks after the spill, BP provided the public with supposedly informed guesses of how much oil was leaking. Many onlookers — some of whom tuned in online to view a “spill cam” trained on the site — began to believe that BP’s numbers were too low.

Initially, BP said 1,000 barrels of oil per day were spilling out. A week later, a National Oceanic and Atmospheric Administration scientist put the number at closer to 5,000 barrels but qualified the estimate as “highly unreliable.”

Rainey developed BP’s next estimate, which mirrored NOAA’s best guess. Prosecutors allege that Rainey’s math actually showed the potential for a much bigger spill but that he withheld that information from investigators. He’s accused of manipulating the numbers to arrive at an estimate near NOAA’s “preliminary and heavily qualified” figure.

Even as BP publicly backed Rainey’s number, its own experts arrived at figures that were up to 30 times higher, prosecutors allege. But Rainey stayed firm, repeating his figure in a May 2010 briefing of a U.S. House subcommittee that was investigating the disaster. Left unchecked, he said, the well could spill at most 60,000 barrels per day in a “worst case” scenario that he labeled impossible.

On May 14, 2010, U.S. Rep. Ed Markey, a Massachusetts Democrat who chaired the subcommittee, sent BP a follow-up letter noting the various higher estimates floated in news reports. Markey expressed concern that BP was deliberately lowballing its estimate, which Markey thought could hamper efforts to plug the well.

“The public needs to know the answers to very basic questions: How much oil is leaking into the Gulf, and how much oil can be expected to end up on our shores and our ocean environment?” Markey wrote.

In response, Rainey submitted a lengthy memo justifying his finding, which prosecutors allege was “false and misleading.” In the memo, they allege, Rainey deliberately gamed the numbers to stay close to his 5,000-barrels-per-day target.

Without conceding that point, Rainey’s lawyers contend that the congressional briefing, Markey’s subsequent letter to BP and Rainey’s response to Markey were not part of any authorized congressional investigation.

Prosecutors say the subcommittee had the same investigative powers as the main congressional committee.

Legal immunity

In an effort to prove the subcommittee exceeded its authority, Rainey last year issued pretrial subpoenas looking for records from nine congressional witnesses, including Markey, now a senator.

The subcommittee voluntarily turned over hundreds of documents. But it agreed to produce dozens of other records only if Rainey stipulated that doing so wouldn’t waive the staffers’ protection under the “speech or debate” clause of the Constitution. That clause gives legal immunity to congressmen and their staffers for things they say, and reports they produce, in the course of carrying out their legislative duties.

Satisfied that the new documents revealed relevant “internal conversations” about the committee’s investigation, Rainey served trial subpoenas to three former congressmen and six staffers, including Markey and former U.S. Rep. Henry Waxman, then the committee’s chairman, who has since retired.

But not surprisingly, the congressional group has resisted the subpoenas, citing the speech or debate clause, and whether they’ll have to testify has yet to be settled. It’s a key aspect of Rainey’s defense that may be cleared up Monday afternoon after oral arguments before Engelhardt.

Rainey’s attorneys argue in court filings that Engelhardt should toss out the obstruction count once again because “congressional witnesses with material, exculpatory evidence, unavailable through other sources, have refused to testify on the ground of privilege, leaving Mr. Rainey unable to present a full defense at trial in violation of his constitutional rights to compulsory process and due process.”

The Office of General Counsel of the U.S. House of Representatives last month responded that Engelhardt should instead quash the nine subpoenas, citing the constitutional privilege afforded by the speech or debate clause.

A difficult tactic

Actually getting lawmakers to testify in third-party criminal cases is rare because of the privilege, legal experts say, but lawyers still try it from time to time. In 2012, attorneys for seven-time Cy Young Award winner Roger Clemens lost a bid to subpoena U.S. Rep. Darrell Issa as a witness in the pitcher’s federal perjury trial.

Clemens was charged with lying under oath about his past steroid use to a U.S. House committee investigating performance-enhancing drugs in baseball. Despite failing to force Issa to testify, Clemens was found not guilty on all counts.

Closer to home, a federal appeals court ruled in 2007 that the FBI violated the congressional privilege during a search of former U.S. Rep. William Jefferson’s legislative office a year earlier. FBI agents reviewed documents before giving Jefferson a chance to determine whether the material was tied to legislative activity, the court found. In 2009, Jefferson was convicted of taking bribes and sentenced to 13 years in prison.

Edward Sherman, a Tulane University law professor who has closely followed the legal fallout from the oil spill, noted that Rainey could have some leeway because the federal government has included on its own witness list four congressional staffers who presumably plan to waive their privilege.

On Friday, Rainey’s attorneys asked Engelhardt to exclude the staffers’ testimony if he won’t hear from the whole group, alleging that the feds are “selectively invoking the speech or debate clause to present a one-sided, unchallengeable account of the alleged inquiry or investigation.”

Sherman agrees.

“They have an interesting argument, in the sense that apparently the prosecution has listed those congressional members as possible witnesses,” Sherman said. “He (Rainey) argues, ‘Well, if they’re going to testify on behalf of the prosecution, then we should be allowed to overcome congressional privilege.’ That’s an interesting point that may distinguish the situations.”

Holding BP to account

Three years ago, then-U.S. Attorney General Eric Holder pledged to “hold accountable those who bore responsibility for this tragedy” as he announced criminal charges against BP as well as Rainey and the two top men on the doomed rig.

Although charges are still pending in four cases, only one has netted a conviction: Anthony Badalamenti, a former Halliburton manager, was sentenced to probation for destroying evidence in the wake of the accident.

Federal prosecutors suffered a blow last year when Duval ordered a new trial for Mix, the first man to be tried over the disaster.

And BP’s top two supervisors onboard the rig when it caught fire and exploded are slated to stand trial next year on lesser charges than those initially handed up in 2012 on the day Holder made his comments.

Supervisors Robert Kaluza and Donald Vidrine are accused of misinterpreting a key safety test and ignoring clear signs that the well was in danger. A federal grand jury charged them with 11 counts apiece of seaman’s manslaughter and involuntary manslaughter, along with violating the federal Clean Water Act.

Duval dismissed the seaman’s manslaughter charges in 2013, ruling that the law is meant to be applied to someone navigating a vessel, not supervisors of a drilling operation, and so did not apply to them. The 5th Circuit backed his ruling earlier this year.

Follow Richard Thompson on Twitter, @rthompsonMSY.