The most striking, if not necessarily the strongest, evidence against Robert Simels, the Manhattan criminal defense attorney on trial for witness tampering and obstruction of justice, has been the recordings made by an undercover witness in which Mr. Simels repeatedly talks of the need to “neutralize,” “eliminate” or even “kill” witnesses.

The case, in which Mr. Simels is accused of colluding with his client, Guyanese drug kingpin Shaheed Khan, to bribe and threaten witnesses, is being heard by Eastern District Judge John Gleeson. If convicted, Mr. Simels faces disbarment and the possibility of life in prison.