At a bridge over the Tiber they were stopped by Cicero's emissaries.

There was a feigned fight, but no blood was shed; and the ambassadors with their letters were brought home to the Consul.
We are astonished at the marvellous folly of these conspirators, so that we could hardly have believed the story had it not been told alike by Cicero and by Sallust, and had not allusion to the details been common among later writers.[204] The ambassadors were taken at the Milvian bridge early on the morning of the 3d of December, and in the course of that day Cicero sent for the leaders of the conspiracy to come to him.
Lentulus, who was then Praetor, Cethegus, Gabinius, and Statilius all obeyed the summons.

They did not know what had occurred, and probably thought that their best hope of safety lay in compliance.

Caeparius was also sent for, but he for the moment escaped--in vain; for before two days were over he had been taken and put to death with the others.
Cicero again called the Senate together, and entered the meeting leading the guilty Praetor by the hand.

Here the offenders were examined and practically acknowledged their guilt.