The trial of John Cort and Brian Farrell was temporarily halted amid claims a policeman was communicating inappropriately with a juror.
It emerged the juror passed a note with her phone number to a police officer for a man she claimed had signalled to her at Nottingham Crown Court.
She did not know the man concerned was a policeman involved in the trial.
Leicestershire Police's professional standards department has launched an investigation into the claims.
Proceedings ground to a halt on the ninth day of the case after a police officer told the court what had happened with the note.
Four days of legal discussions followed which ultimately meant the woman was discharged from the jury along with another juror who did not want to continue.
With the jury reduced to 10, it was agreed by the judge, prosecution and defence counsel that the case could continue.
Mrs Justice Linda Dobbs said the force could start its investigations, involving looking at the note the juror had written and examining CCTV of the court building, but could not interview any jury members until the end of the trial.
After the trial ended temporary Deputy Chief Constable Gordon Fraser said: "The force demands the highest standard of professionalism from all staff and will thoroughly investigate any allegations of inappropriate behaviour."
On Friday, John Cort, 54, of Rutland Street, Leicester, was jailed for life to serve a minimum term of 29 years and hit man Brian Farrell, 37, of Queensborough Terrace, west London, was jailed for a minimum term 27 years for the murder of Vina Patel.