In Harper Lee's To Kill a Mockingbird, the element that provides most of the apprehension at the end of Chapter 27 is found in the literary device of foreshadowing which we, as readers, do not recognize until the event referred to in the future has taken place.
Dr. L. Kip Wheeler defines foreshadowing as:
Suggesting, hinting, indicating, or showing what will occur later in a narrative. Foreshadowing often provides hints about what will happen next...Often this foreshadowing takes the form of a noteworthy coincidence or appears in a verbal echo of dialogue.
There are several examples of foreshadowing in this chapter. First, we can sense, in hindsight, that Bob Ewell must have been extremely angry, even though the jury found in his favor in Tom Robinson's court case.
The first thing was that Mr. Bob Ewell acquired and lost a job in a matter of days...I suppose his brief burst of fame brought on a briefer burst of industry, but his job lasted only as long as his notoriety: Mr. Ewell found himself as forgotten as Tom Robinson...Ruth Jones, the welfare lady, said Mr. Ewell openly accused Atticus of getting his job.
Atticus tells Ruth not to worry; if Bob Ewell has a problem, he can come to see Atticus at his office. However, now knowing that Bob Ewell was a coward with murder on his mind, he would never have tried to speak in a civilized way to Atticus, especially when his accusations meant nothing. Bob Ewell lost the job because he was lazy, but he wanted to blame someone else.
The fact that another member of the court is nearly accosted when someone tries to break into his home gives the audience the idea that there is something going on. At the end of the book, we can assume it had everything to do with the court case and Bob Ewell, a twisted character at best.
Next, Link Deas (Tom's old boss) give Helen Robinson a job, but she cannot walk the public road past the Ewell's property, or they throw things at her. Link Deas threatens Bob Ewell once; then when Helen takes the public road:
...when she was a few yards beyond the Ewell house, she looked around and saw Mr. Ewell walking behind her...[his] soft voice behind her, crooning foul words.
Deas puts pressure on Ewell again, this time threatening to get the "Ladies' Law" after him. This time, Ewell backs off.
Even Aunt Alexandra is worried:
I don't like it, Atticus, I don't like it at all...That man seems to have a permanent running grudge against everybody connected with that case.
Although Atticus tries to brush it aside, he has to admit Judge Taylor made Ewell look like a fool in court—like a "three-legged chicken or a square egg."
When Scout describes her costume for the pageant, she points out that she cannot move her arms in it, and someone needs to take it off of her, for she cannot do it herself...we see this in the attack.
Later, Aunt Alexandra opens her mouth to say something: nothing comes out...unusual for Scout's aunt. When Scout asks her if she is all right, she responds:
Oh nothing, nothing...somebody just walked over my grave.
And if these examples are not enough, Scout's famous line is enough to give one goosebumps, especially after reading the story's ending:
Thus began our longest journey together.
This also is foreshadowing; it sounds portentous, as if there is more at stake here than walking to and from the schoolhouse: and that is exactly what it ends up meaning...it is much more than a simple walk for these kids: it is a "matter of life and death."