Sometime before April 7, ExxonMobil will finally tell regulators and the public why its 1940s-era Pegasus oil pipeline split open in Mayflower, Ark. last March, spilling thick Canadian dilbit into a neighborhood and nearby cove.

Will Exxon just send out a statement announcing its conclusions about the cause or causes of the Pegasus spill? Or will it also make public the details and supporting evidence behind its determination? If Exxon doesn't provide those details, will they be made available by the Pipeline and Hazardous Materials Safety Administration (PHMSA), which regulates most U.S. pipelines?

Actions to date aren't encouraging, according to some pipeline experts and Arkansas officials. "It's been a constant process of trying to get information, trying to get data, trying to evaluate the tools and technology and processes that Exxon uses to ensure the integrity and safety of this pipeline," said John Tynan former watershed protection manager for Central Arkansas Water (CAW) and now its director of public affairs.

The pipeline, which stretches from Patoka, Ill. to Nederland, Texas, burst open about eight miles from CAW's Lake Maumelle Watershed, and more than 18 miles of the pipeline run through it. Since the watershed helps provide the water that CAW supplies to 400,000 people, the water utility has asked Exxon for detailed information about last year's failure as well as the pipeline's condition within the watershed.

To get that information "there have been a number of hoops and hurdles we've had to jump through ... namely having to sign a Confidentiality agreement that Exxon requested because of their concerns over proprietary or security-related issues," Tynan said. The CAW signed it, reluctantly.

The secrecy surrounding the Pegasus spill has been there from the beginning, frustrating reporters, homeowners, lawmakers, local officials and people elsewhere along the 858-mile pipeline, which starts in Patoka, Ill. and ends in Nederland, Texas.

More: One Year After Exxon's Arkansas Spill, 8 Crucial Questions Still Unanswered

First, the Federal Aviation Administration issued a temporary flight restriction—said to have been sought by an Exxon representative—that prohibited flights below 1,000 feet within a five-mile radius of the Mayflower spill. The restrictions were lifted after a few days, but they were criticized as excessive and serving corporate interests because it handicapped news coverage and could have interfered with efforts to sample air conditions below that elevation, according to the FAA watchdog site aireform.com.

Other difficulties with getting spill-related information to the public followed:

Several days after the Mayflower spill, an Exxon employee threatened to have an InsideClimate News reporter arrested for criminal trespass because she went to the spill command center seeking public affairs contacts for the government agencies on site.

Exxon released a statement about the conclusions contained in a company-funded post-spill metallurgical report, but refused to provide copies of the report itself. Congressman Tim Griffin of Arkansas, who has called the secrecy surrounding the pipeline spill "ridiculous," posted it on his website. PHMSA later followed suit.

Exxon refused to release data from a series of inspections and tests conducted on the Pegasus, claiming a trade secret exemption. Those reports were eventually made public by PHMSA.

Exxon's 2013 emergency response plan for the Pegasus had to be obtained with a Freedom of Information Act request. When it was released two months later, it was so heavily redacted that in many places, Exxon's corporate logo was partially blacked out. More than 100 pages of the 290-page document were entirely blotted out.

Neither Exxon nor PHMSA has released testing data for the southern leg of the Pegasus, which may have seam-rupture tendencies similar to those of the northern segment. Exxon hopes to reopen that part of the pipeline soon.

The Pegasus information page set up by PHMSA has not been kept up to date, and other documents, such as Exxon's restart plan for the Texas portion of the Pegasus, have not been made public.

Some have attributed the lack of information to a penchant for secrecy and a bunker mentality that they say has been pervasive within Exxon at least since the Exxon Valdez tanker disaster in Alaska 25 years ago.

Richard Kuprewicz, a consultant advising the Arkansas water utility on the Pegasus case, has been critical of Exxon's tight hold on information. He had this to say about withholding such things as Pegasus restart plans: "If Exxon thinks it's just between them and PHMSA, that could be a problem."

Exxon spokesman Aaron Stryk has been answering e-mailed questions from InsideClimate News and others, but has referred requests for the Pegasus startup plan as well as other information and document requests to PHMSA.

"As we have stated before, while the investigation is ongoing, it is PHMSA that defines the timing, type and amount of investigative information to share with appropriate stakeholders and the general public," Stryk said.

Others have suggested that PHMSA is underfunded and may have grown too cozy with the industry it regulates. Because of those things, observers have said, the agency doesn't typically challenge company claims that test data, spill response plans, restart plans and other information are proprietary or a security threat. If such documents are released by PHMSA at all, it is often with extensive redaction.

Compared to 15 years ago, "PHMSA has provided a lot more information automatically on their website about incidents and those types of things," said Carl Weimer, executive director of the Pipeline Safety Trust, a watchdog group that grew out of the 1999 Bellingham, Wash. pipeline explosion that killed three people. "But specific information about investigations or records that the companies provide [to] them still are [both] very hard to get hold of."

Steven Da Silva of the Texas-based Safe Community Alliance is one of many people who have grown frustrated by the lack of information about the pipeline.

"I would like to believe that somewhere ... there would be the realization that being open with what's being done to relieve or assuage the fears of the residents along the pipeline path would be something that they [should] take on," said Da Silva, whose group opposes restarting the Pegasus. "It's mind-boggling that they don't take that initiative. Everything's an afterthought if it doesn't have to do directly with getting this [dilbit] to market."

PHMSA and the oil industry already have new high-profile cases where they could do things differently and improve the flow of information to the public. In the last few weeks alone, a pipeline spilled more than 20,000 gallons of crude oil into an Ohio nature preserve; and a barge collision dumped an estimated 168,000 gallons of thick bunker fuel oil into the Houston channel, temporarily closing it to maritime traffic.