diff --git a/internal/keyfinder.go b/internal/keyfinder.go
index 5d0aa00..3e6e1cd 100644
--- a/internal/keyfinder.go
+++ b/internal/keyfinder.go
@@ -13,43 +13,53 @@ import (
 // NER is the error message returned when the input has less fields than the KeyFinder is configured for.
 const NER = "not enough bytes in record"
 
-// KeyFinder is a slice of small integers representing field numbers; 1-based on the command line, 0-based here.
-type KeyFinder []uint
+// KeyFinder extracts a key based on the specified fields from a record. fields is a slice of small integers
+// representing field numbers; 1-based on the command line, 0-based here.
+type KeyFinder struct {
+	fields []uint
+	key    []byte
+}
 
 // NewKeyFinder creates a new key finder with the supplied field numbers, the input should be 1 based.
+// KeyFinder is not threadsafe, you should Clone it for each goroutine that uses it.
 func NewKeyFinder(keys []uint) *KeyFinder {
-	if keys == nil {
-		return nil
+	kf := KeyFinder{
+		key: make([]byte, 0, 128),
 	}
-
-	var kf KeyFinder
 	for _, knum := range keys {
-		kf = append(kf, knum-1)
+		kf.fields = append(kf.fields, knum-1)
 	}
 	return &kf
 }
 
+// Clone returns a new KeyFinder with the same configuration. Each goroutine should use its own
+// KeyFinder instance.
+func (kf *KeyFinder) Clone() *KeyFinder {
+	return &KeyFinder{
+		fields: kf.fields,
+		key:    make([]byte, 0, 128),
+	}
+}
+
 // GetKey extracts a key from the supplied record. This is applied to every record,
-// so efficiency matters
+// so efficiency matters.
 func (kf *KeyFinder) GetKey(record []byte) ([]byte, error) {
 	// if there are no keyfinders just return the record, minus any trailing newlines
-	if kf == nil || len(*kf) == 0 {
+	if len(kf.fields) == 0 {
 		if record[len(record)-1] == '\n' {
 			record = record[0 : len(record)-1]
 		}
-		// Make a copy of record as record is a pointing at a buffer that will be reused
-		// and the caller is going to hang onto this.
-		return append([]byte(nil), record...), nil
+		return record, nil
 	}
 
 	var err error
-	key := make([]byte, 0, 100)
+	kf.key = kf.key[:0]
 	field := 0
 	index := 0
 	first := true
 
 	// for each field in the key
-	for _, keyField := range *kf {
+	for _, keyField := range kf.fields {
 
 		// bypass fields before the one we want
 		for field < int(keyField) {
@@ -64,18 +74,18 @@ func (kf *KeyFinder) GetKey(record []byte) ([]byte, error) {
 		if first {
 			first = false
 		} else {
-			key = append(key, ' ')
+			kf.key = append(kf.key, ' ')
 		}
 
 		// attach desired field to key
-		key, index, err = gather(key, record, index)
+		kf.key, index, err = gather(kf.key, record, index)
 		if err != nil {
 			return nil, err
 		}
 
 		field++
 	}
-	return key, err
+	return kf.key, err
 }
 
 // pull in the bytes from a desired field
diff --git a/internal/segmenter.go b/internal/segmenter.go
index 519fd07..7fa428a 100644
--- a/internal/segmenter.go
+++ b/internal/segmenter.go
@@ -126,10 +126,12 @@ func readAll(s *Segment, filter *Filters, kf *KeyFinder, reportCh chan segmentRe
 	reader := bufio.NewReaderSize(s.file, 16*1024)
 	current := s.start
 	counters := newSegmentCounter()
+	kf = kf.Clone()
 	for current < s.end {
-		// ReadSlice results are only valid until the next call to Read, so we
-		// to be careful about how long we hang onto the record slice.
-		// In this case GetKey needs to never return a direct subslice of the record.
+		// ReadSlice results are only valid until the next call to Read, so we need
+		// to be careful about how long we hang onto the record slice. The SegmentCounter
+		// is the only thing that holds onto data from record, and it has to make a copy
+		// anyway when it constructs its string key. So this is safe.
 		record, err := reader.ReadSlice('\n')
 		// ReadSlice returns an error if a line doesn't fit in its buffer. We
 		// deal with that by switching to ReadBytes to get the remainder of the line.
diff --git a/internal/segmenter_test.go b/internal/segmenter_test.go
index d72ae6e..775057b 100644
--- a/internal/segmenter_test.go
+++ b/internal/segmenter_test.go
@@ -61,7 +61,7 @@ func TestReadAll(t *testing.T) {
 
 func TestReadAllLongLine(t *testing.T) {
 	counter := NewCounter(10)
-	err := ReadFileInSegments("../test/data/long_lines", &Filters{}, counter, nil, 1)
+	err := ReadFileInSegments("../test/data/long_lines", &Filters{}, counter, NewKeyFinder(nil), 1)
 	if err != nil {
 		t.Fatalf("Failed to process file %v", err)
 	}