Police arrested three people in Spalding after finding a man with a serious head injury.
Officers attended reports of an assault just after 7.20am on Wednesday, February 5 on Cedar Court.
They are still trying to identify the victim and have appealed to witnesses to what they believe was an assault.
One man, 43, and two women, 40 and 18, have been arrested on suspicion of carrying out the assault.
Two of them remain in custody but the 18-year-old has now been released without any further action as investigations continue.
Anyone with information has been urged to call 101 and quote incident 63 of February 5.