OKLAHOMA CITY - The man heading up an investigation into last month's execution of an Oklahoma inmate says the probe into what went wrong could be wrapped up in a "matter of weeks."

State Department of Public Safety Commissioner Michael Thompson said in a statement Friday that significant progress is being made in the investigation into the April 29, 2104 execution of Clayton Lockett.

"In the past four weeks since we have had this assignment, our team has made tremendous progress," said Michael Thompson.

"We are reasonably confident that we can finish the investigation in a matter of weeks instead of months. We are close to concluding interviews of all key individuals and compiling our findings while we await the results of the autopsy. We are striving to ensure that when we turn our findings over to the Governor, she will have a complete, accurate, and unbiased summary of our investigation in addition to our recommendations. The findings of the report will be made public when it is turned over to the Governor."

Prison officials have said Lockett's vein collapsed during his lethal injection, prompting prison officials to halt the execution. He later died of a heart attack.

Gov. Mary Fallin ordered an investigation into the execution, which was carried out using a new three-drug combination that had never before been tried in Oklahoma.

DPS officials initially said the probe could take up to four months.

The Associated Press contributed to this report.