HOUSTON—A decision on whether to approve a much-criticized plea deal that settles criminal conduct related to a deadly explosion at a BP PLC plant won’t come until October at the earliest, a federal judge said Wednesday.

The possible October ruling by U.S. District Judge Lee Rosenthal will mean that a year will have passed since the agreement between BP and federal prosecutors was announced.

The agreement, which has a BP subsidiary pleading guilty to a violation of the Clean Air Act, calls for a $50 million fine and sentences the oil giant to three years’ probation for its role in the Texas City blast that killed 15 people and injured more than 170.

The victims think the fine is low and that the terms of probation within the plea deal do not provide for an independent watchdog to monitor whether BP would meet its safety obligations at the refinery.

Federal prosecutors and BP have defended the plea agreement, saying it’s the harshest option available in assessing criminal punishment. A congressional committee is investigating the deal.

Rosenthal said she needed two more sets of information before she can decide whether to accept the plea agreement.

She requested that attorneys for blast victims submit a report from an expert witness detailing why they think BP is not currently meeting safety requirements outlined in agreements the oil giant entered into with state and federal authorities after the blast.

The judge also asked blast victims attorneys to provide detailed information about actual economic losses their clients have suffered so she can determine if these losses are greater than the $50 million fine that is being proposed.

Attorneys for blast victims were directed to gather this information on economic losses from a representative group of 50 victims, made up of those 15 who were killed and 35 others who suffered the most serious injuries or property losses as a result of the blast.

Federal prosecutors and attorneys for BP questioned whether such information was needed to make a decision.

BP formally entered its guilty plea during a February court hearing. A decision by Rosenthal on the deal’s fate was pending when blast victims appealed.

The case made it to the U.S. Supreme Court, which earlier this month denied a request from blast victims to delay a decision on the settlement.

Attorneys for blast victims say the agreement should be voided because their clients’ rights under the Crime Victims’ Rights Act were violated when they weren’t consulted about the plea deal.

The case ended up before the Supreme Court after a three-judge panel of the 5th U.S. Circuit Court of Appeals ruled in May that prosecutors violated the rights of blast victims when they didn’t consult them about the plea deal before it was announced late last year. Rosenthal had previously ruled that prosecutors hadn’t violated blast victims’ rights.