Police in Fort Worth, Texas, have released a letter that may contain a cryptic clue into the unsolved murder of a 17-year-old girl in 1974.

According to the Dallas Morning News, Carla Walker was raped and brutally murdered in February 1974. Her body was found a few days after an unknown assailant pulled her out of the passenger seat of her boyfriend’s car, and hit the boyfriend in the head with a pistol.

“She was such a sweet girl,” the boyfriend, Rodney McCoy, told WFAA-TV in an interview last year. “I remember we were in the front seat of the car. Her back was against the passenger door. She was falling out. I went to grab her, and he started beating me over the head with a pistol.”

It is not clear exactly when police received the handwritten letter released this week. The letter, also shared on the department’s Facebook page, includes a sentence that is partially redacted, and reads “killd [sic] Carla Walker in Benbrook” immediately after the redacted portion of the sentence.

In a postscript, the letter reads: “It is hard to say but it is true.” It also contains, twice, the phrase “Sign 10100,” which, according to the Dallas Morning News report, investigators believe may be a reference to a police code for a dead body.

The Fort Worth police are seeking more information about the contents of the letter.

“The author of the letter, or anyone who knows the author of the letter, is asked to call 817-392-4307,” the Facebook post reads.