U.S. Drug Enforcement Administration acting administrator Robert Patterson, left, speaks in Omaha on June 7, 2018, in front of Neb. Gov. Pete Ricketts. Patterson told DEA staff on Monday that he plans to retire. (Nati Harnik/AP)

The acting head of the Drug Enforcement Administration told staff Monday he is retiring, saying that running the agency as a temporary fill-in had become “increasingly challenging.”

Robert W. Patterson, who has worked at the DEA for 30 years, sent an email to employees Monday afternoon saying he will retire in about two weeks.

Patterson said he “realized that the administrator of the DEA needs to decide and address priorities for years into the future — something which has become increasingly challenging in an acting capacity.” His email was reviewed by The Washington Post.

It was not immediately clear who would succeed Patterson as acting DEA administrator.

Patterson became the agency’s acting head in October, following the departure of Chuck Rosenberg, who had also served as an acting, rather than Senate-confirmed, head of the agency. Rosenberg’s departure came after months of tension between him and Attorney General Jeff Sessions over marijuana research policy and the Trump administration’s focus on pursuing the MS-13 street gang, rather than sophisticated drug cartels.

Rosenberg had also put himself at odds with the president, emailing staff members that Trump had “condoned police misconduct” in remarking to officers in Long Island that they need not protect suspects’ heads when loading them into police vehicles.

“Since taking on the role of acting administrator, I have known that a permanent replacement would eventually be named. As such, I took each day as a gift, and with that mentality tried my best to keep the agency moving forward,” Patterson wrote in the message.

Patterson, a New Jersey native, began his DEA career in the New York office, pursuing racketeering cases.

Julie Tate contributed to this report.