In fact, it was almost demonstrable that an alert criminal like the man they were pursuing--if he really were the ally of Hunter's slayers--could hardly have failed to realize much earlier that he was being followed.
Moreover, being an expert motorist, he would know that the car in the rear could not only hold him in the race but close up with him whenever its occupants were so minded.

He would not be lulled into false security by the present widening of the gap, because that was an obvious maneuver due to altered circumstances.

In a word, there was now no hope or prospect of running him to earth at a rendezvous, but, giving him credit for the possession and use of a criminal's brains, it became an urgent matter to overtake him and compel a halt by deliberately blocking the way.
They debated the point fully, and Devar was about to tell Brodie to act when the gray car disappeared.
Not wishing to interfere at a critical moment, Devar drew back from the window.

Brodie spurted down a hill and along a short level lined with suburban villas; he slowed to take a sharp corner, and the car ran along a winding lane which could lead nowhere but to the water's edge.
It was pitch dark, and a mist from the Hudson filled the valley.
Common sense urged a careful pace, because it had never been possible to stop and adjust the powerful headlights, while the luminous haze of an occasional street lamp served only to reveal the narrowness of the road and the presence of shacks and warehouses.
The descent was fairly steep, so Brodie shut off the engine, and the big car crept on with a stealthy and noiseless rapidity which seemed to betoken an actual sense of danger.
Suddenly they heard a loud splash, accompanied by a muffled explosion, and McCulloch relieved his feelings by a few words, the use of which is expressly forbidden by the police manual.

But their purport was ridiculously clear; the gray car had plunged into the Hudson, and who could tell whether or not Anatole had gone with it?
Curtis was the first to adopt a definite line of reasoning: he assumed command now with the confidence of one accustomed to be in tight places and to depend on his own wits for extrication.
"Go forward slowly until the buildings stop, Brodie," he said, for the two front windows were lowered, and the three men were crowded at them.
"That fellow knew exactly where he was going.