The US embassy in Mexico City said in a statement that the consulate in Juarez, across the border from El Paso in Texas, would remain closed until a security review is completed.
A US official said the consulate had been shut after receiving a 'credible threat'.
It closed briefly in March after three people connected to the consulate were murdered by drug hitmen.
Ciudad Juarez has become one of the world's most violent cities, with some 6,000 people killed over the past two-and-a-half years as rival drug cartels fight over smuggling routes into the US.
Separately, two Frenchmen working at the US Embassy in Paris have undergone medical check-ups after handling a suspect letter.
Initial tests suggest that the incident was a false alarm.
An embassy spokesman said 'I can confirm that there was a letter in the mailroom that was deemed suspicious by the handlers. The employees were taken to medical personnel for attention, we do not know if it was poison. We do not have any confirmation that they were hurt, the incident involved two locally employed male French citizens who had been taken for check-ups.'
Post rooms at US diplomatic facilities worldwide are always on the lookout for suspicious packages, amid fears that extremists might try to send in bombs or toxic materials through the post.
