The shooting happened during a traffic check near Kusel at about 4.20am on Monday, police in Kaiserslautern said in a statement.
The officers radioed that shots were being fired, spokesman Bernhard Christian Erfort told n-tv television.
But reinforcements who arrived at the scene were unable to help the 24-year-old woman and 29-year-old man.
Erfort said he did not know whether the officers had seen something particular about the assailants' vehicle that they wanted to check or whether it was just a routine check.
German news agency DPA, citing unidentified security sources, said that the officers reported finding dead game in the car before the shooting started.
The Bild daily also reported that find, without citing sources.
Police had no description of the perpetrators, the car they used or what direction they fled in.
A manhunt has been extended to the neighbouring German state of Saarland, police said.
They called on drivers in the Kusel area not to pick up hitchhikers and warned that at least one suspect is armed.
The younger officer killed in Monday's incident was still studying at a police academy, the GdP police union said.
"Regardless of the motive, this crime is reminiscent of an execution and shows that the police risk their lives for our security every day," Interior Minister Nancy Faeser tweeted.
She said that "we will do everything" to catch the perpetrators.
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