Mary Bosanquet, in The Life and Death of Dietrich Bonhoeffer, gives the details of that day…

…the next day, April 5th (1943), at midday, when Dietrich tried to speak to (his sister) Christine (von Dohnanyi) by telephone in her house at Sakrow, the telephone was answered by an unknown man’s voice.  Dietrich guessed the truth at once: that Hans and Christine had been arrested and that the secret police were ransacking the house.

Without saying anything to his parents, he went next door to his sister Ursula Schleicher and asked her to prepare him a large meal.  Then he went up to his attic bedroom in his parents’ house, checked it once more for incriminating papers, and left about, not too conspicuously, one or two specially fabricated notes which he wished the Gestapo to find.

Then he returned to the Schleichers’ house and waited there with the Schleichers and Bethge.  About four o’clock his father looked in: “Dietrich, two men want to see you in your room!”  The two men were Roeder, Chief Investigator for the Air Force, and Sonderegger, a member of the Gestapo.  They said little and without producing a search warrent or any formal notice of arrest, they ordered Bonhoeffer to accompnay them to their car.

Quietly, undramatically, the black Mercedes drove away.  Soon Dietrich saw the gloomy facade of Tegel Military prison rising in front of them, a frowning cliff of masonry, pockmarked with barred windows.  The car halted.  They stood at the entrance.  Roeder said a word to the warder who approached.

Without ceremony, Bonhoeffer was bundled over the threshold.  The iron gates crashed to behing him.

He was in prison (244).