Arrests and investigations made in connection with travelers checks' offenses indicate that generally professional criminals are responsible for their fraudulent use. Offenders will steal passports, identity cards, and other identity documents from consulates, official institutions, and private persons and then fraudulently alter these documents so they can be used when cashing the checks. Travelers checks that have been lost or stolen are sold for a percentage of their face value. Offenders have various tricks for appearing to sign the check in the presence of the receiver to duplicate the signature of the original owner; for example, signing some checks which have been acquired legally, dropping them, and replacing them with those acquired illegally. Stolen blank travelers checks are usually acquired through armed robberies of banks and travel agencies or thefts of parcels or packages at post offices, airports, or railway stations, or during transport. When genuine travelers checks are fraudulently altered, they are cashed in the same way as stolen bank checks. The difficulties encountered by police when investigating this type of offense stem from the widespread use of false papers, the tendency of swindlers to stay only short periods in one place, and the ease with which checks can be changed, primarily because the sheer volume of banking operations means that the institutions accepting them cannot make the necessary verifications in the countless cases reported by the issuing institutions.