An Old Bailey judge temporarily halted a trial to reassure jurors after claims of intimidation.
The panel trying the case of three men accused of a shooting in which a five-year-old girl was left paralysed had raised concerns with Judge Martin Stephens QC.
Details of what had happened were given in a note which was not read out in open court.
This afternoon the judge asked them to continue in the "difficult and demanding case" but to "monitor the situation carefully and please report any further matter which causes you any concern immediately and steps will be taken".
He said that while the panel were "not unduly concerned" by what had happened, if any further incidents were raised he would bar the public from the rest of the trial.
Judge Stephens said if there were further reports: "I shall want immediate steps to be taken starting with the arrest of anyone suspected of such conduct."
He also stressed that any event could not be taken to reflect badly on the three defendants Kazeem Kolawole, 19, Anthony McCalla, 19, and Nathaniel Grant, 21.
The three men are on trial for causing grievous bodily harm to Thusha Kamaleswaran and a shopper who was also shot, Roshan Selvakumar.
They are also accused of the attempted murder of another man, Roshaun Bryan, and possession of a firearm with intent. They deny all the charges.
The jury was sent home until Friday, when the prosecution will continue.