WASHINGTON — The FBI said Wednesday that it will not publicly disclose the method that allowed it to access a locked iPhone used by one of the San Bernardino attackers, saying it lacks enough “technical information” about the software vulnerability that was exploited.

The decision resolves one of the thorniest questions that has confronted the federal government since it revealed last month, with minimal details, that an unidentified third party had come forward with a successful method for opening the phone. The FBI did not say how it had obtained access, leaving manufacturer Apple Inc. in the dark about how it was done.

The new announcement means that details of how the outside entity and the FBI managed to bypass the digital locks on the phone without help from Apple will remain secret, frustrating public efforts to understand the vulnerability that was detected and potentially complicating efforts to fix it.

In a statement Wednesday, FBI official Amy Hess said that although the FBI had purchased the method to access the phone – FBI Director James Comey suggested last week it had paid more than $1 million – the agency did not “purchase the rights to technical details about how the method functions, or the nature and extent of any vulnerability upon which the method may rely in order to operate.”

The FBI’s explanation raises the possibility that it applied a purchased exploit against what it described as a potentially key piece of evidence in a sensational terrorism investigation without knowing the full technical details of what it was doing to that iPhone.

The government has for years recommended that security researchers work cooperatively and confidentially with software manufacturers before revealing that a product might be susceptible to hackers. The Obama administration has said that while disclosing a software vulnerability can weaken an opportunity to gather intelligence, leaving unprotected Internet users vulnerable to intrusions is not ideal either.

An interagency federal government effort known as the vulnerabilities equities process is responsible for reviewing such defects and weighing the pros and cons of disclosing them, taking into account whether the vulnerability can be fixed, whether it poses a significant risk if left unpatched and how much harm it could cause if discovered by an adversary.

Hess, the executive assistant director of the FBI’s science and technology branch, said Wednesday the FBI did not have enough technical details about the vulnerability to submit it to that process.

“By necessity, that process requires significant technical insight into a vulnerability. The VEP cannot perform its function without sufficient detail about the nature and extent of a vulnerability,” she said.