In part two of a two-part series on the work of the world-leading forensic science laboratory in Huntingdon, ANGELA SINGER looks at DNA, documents and drugs. A MAN who wrote a threatening letter to a chief constable believed he would be above the law. B

MACABRE: This threatening letter included primitive drawings.

In part two of a two-part series on the work of the world-leading forensic science laboratory in Huntingdon, ANGELA SINGER looks at DNA, documents and drugs.

A MAN who wrote a threatening letter to a chief constable believed he would be above the law.

But he was caught because he also wrote a (non-threatening) letter to his dentist on the same notepad. The impression of the letter to the dentist was found when the chief constable's letter was examined under "ESDA" (electrostatic detection apparatus).

The police were able to go the health centre, match the handwriting and identify the offender from his dentist's files.

Examining handwriting and the impressions left on paper is part of the work of the Forensic Science Service at its laboratory in Hinchingbrooke, Huntingdon.

"This is not graphology. We are not looking at what the handwriting says about the character of the person," said "Mary", who did not wish to be further identified because of the sensitive nature of the work. "We deal with the way the letters are constructed, the pen patterns, the shapes and the proportions."

Handwriting is examined under a microscope to see how the letters are formed. The documents section of the laboratory will also look at blotches or missing parts of a letter formed by the blocked nozzles of an ink jet printer. The department can also - though it takes time - stick back together documents which have been shredded.

Looking for forgeries, the lab will identify items that have been added or chemically erased and break down ink to see whether a handwritten or typed entry has been added to a document after the original details were produced. Special equipment and techniques will distinguish ink and paper that looks similar to the naked eye - but is actually quite different - and identify forged stamp marks, linking them to the stamps used to produce them.

Mary said: "We analyse the inks, paper and glue to see how they are used in the production of a document and we can establish the sequence of events to see whether a disputed document has been made at the same time as the others around it. Documents can be linked to the office machine, the printer, photocopier or fax machine that produced it."

In another letter on the scientists'books the sender threatened to shoot the recipient unless he was given£60,000, the writer had provided even more evidence by taking the trouble to draw - and colour in - a picture of a man holding a gun and another lying dead on the floor.

Because the sender used bright colours and the picture was primitive, the macabre picture looked like a child's drawing.

n There could well have been DNA also found on a threatening letter or forgery. Pinpointing DNA is a massive and expanding part of the work of the Forensic Science Service.

Until 1999, DNA found needed to be about the size of a five pence piece to be useful in detection work.

Improved techniques mean that the system can now work with traces invisible to the human eye.

Kam Sandhu, 25, is a DNA analyst with a degree in biomolecular science from John Moores University in Liverpool, who joined the FSS as a new graduate.

Her job is to identify and code DNA.

"DNA examination techniques are now so sophisticated that, if two people have shared a cigarette and both left their DNA on the stub, the two can be separated out. Unless you are an identical twin, every person's DNA is different. However, for identification purposes. It is purely a code number. DNA alone does not tell you anything about a person except whether they are male or female.