The author of a pioneering blog about Mexico's drug war has said that she has fled the country and that her blog partner has gone missing.

The young woman, using her pseudonym Lucy, said her colleague phoned her last week to say a single word – "run" – and then vanished, prompting her to flee to the United States and then Spain.

"I'm trying to think positively but I'm scared something terrible has happened. 'Run' was our codeword for when something was very wrong. We had never used it before."

Blog del Narco is an internet sensation which has chronicled Mexico's drug war with graphic images and shocking stories few others dare show. It has been a must-read for authorities, drug gangs and millions of ordinary people.

The anonymous author was amystery until last month when she revealed to the Guardian and Texas Observer she was a woman, not a man as previously assumed, and that with her colleague she had written a book, Dying for the Truth: Undercover Inside Mexico's Violent Drug War.

Dying for the Truth: Mexico drug wars

The revelation caused a stir but the duo continued as normal, Lucy, a journalist, writing and editing the site and her partner, a male friend aged 27 who lived in a different city in northern Mexico, managing the technical side.

On 5 May he phoned her. "He just said 'run'. Then hung up. It was our code word for extreme situations, our last resort, but until then we had never used it. I called him back but there was no answer.