Delhi Police has served a notice to Congress MP Shashi Tharoor asking him to join investigation into the mysterious death of his wife Sunanda Pushkar.Sources said that Tharoor, who is in Kerala, has been asked to join probe at the earliest.”A legal notice has been issued to Tharoor to join the murder probe. The questioning will be held under section 160 CrPC,” said a senior an official.Delhi Police had registered a case of murder a few days ago, following the final report from the board of doctors which conducted post-mortem on Pushkar.An SIT has also been formed by the police to probe the case. On Tuesday, Delhi Police had registered a murder case on the basis of a medical report that concluded her death was unnatural and due to poisoning but nobody has been named as a suspect as yet.Also read: Sunanda Pushkar case: Police trying to set me up, claims Shashi TharoorNearly a year after 51-year-old Sunanda was found dead in a luxury hotel here on January 17, Delhi Police filed the case of murder under section 302 of the IPC but no one has been named in the FIR.The medical board of AIIMS submitted a fresh report to the police on December 29 and stated that it was the injection, suspected of polonium, that may have led to Sunanda’s death. The report clearly stated it was an unnatural death.In its report AIIMS medical board had said, “The cause of death in this case is poisoning. Viscera are positive for ethyl alcohol, caffeine, acetaminophen and cotinine.” From the medical document made available to them the panel has also concluded that Sunanda did not have cardiac problem. She was not suffering from any diseases like hypertension, diabetes or tuberculosis.The panel has concluded that Sunanda was neither ill nor had any disease prior to her death. She was a normal healthy individual.Also read: Sunanda Pushkar murder: Pakistan journalist Mehr Tarar says she is ready to answer any questionThe medical board had examined all the injuries on her body out of which ‘injury number 10’ has been termed as a mark caused by the needle of a syringe.The team has also dismissed the theory that Sunanda died of an overdose of Alprazolam as the viscera report is negative for the presence of the substance.Sunanda was found dead in a 5-star hotel in South Delhi on the night of January 17, a day after her Twitter spat with Pakistani journalist Mehr Tarar over an alleged affair with Tharoor.The probe into the case was handed over to the Crime Branch on January 23. However, the case was transferred back to the South District police two days later on January 25.