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 a mystery 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. I emailed him, tried Skype and WhatsApp, but nothing. Nothing."

Lucy, speaking to the Guardian via Skype this week, cried several times. She said she was speaking from an undisclosed location in Spain and that she was alone, lonely and frightened.

Her account could not be independently verified but a US-based intermediary who is also in contact with Lucy backed up her story. Adam Parfrey, head of the Washington-based publisher Feral House, which published Dying for the Truth, said he was not in direct contact with the authors but had heard a rumour one had disappeared. "I hope it's not true."

Lucy said that after receiving the phone call she immediately moved to another part of her home city, in northern Mexico, and prepared to flee. She sold some of her great-grandmother's jewellery, took a bus to the border and legally entered the US on foot. "I had all the correct papers." Hours later she was on a flight to Spain, she said. "It's further away. It feels safer."

She has not posted on the blog since 3 May, she said, but technical issues related to previous cyber-attacks meant it appeared on the site on 8 May. She has no immediate plans to resume blogging.

She is in a boarding house and has enough money to last a few months, she said, but has no friends or contacts in Spain.

Her biggest fear is she will see her colleague appear on a video of the type that frequently appeared on their blog: battered, interrogated, gazing into the camera, knowing a terrible fate awaits. Some victims have been tortured and beheaded on camera. "I don't want to think the worst but I can't help it."

More than 70,000 people have died in the past six years, including dozens of journalists, as cartels battle each other and state forces. Tens of thousands more have vanished.

Blog del Narco helped fill a vacuum left by cowed mainstream media organisations which often could not report roadblocks, shootouts and kidnappings.

Over time it acquired multiple sources, including drug gangs, and drew more than 3m hits monthly. It provided bulletins, pictures and video of abductions, shootouts, executions and the discovery of bodies as well as severed human heads, limbs and torsos.

Some critics said it provided a platform to cartels, others complained the blog cut and pasted reports from other sources without attribution.

The site has come under repeated cyber-attack – the government was more aggressive than narcos in this regard, Lucy said. The biggest risk was being identified and abducted, either by narcos or government forces who have been accused of multiple abuses.

A young couple who contributed material to the blog was abducted, tortured and disembowelled in 2011 in the state of Tamaulipas. A sign next to the bodies said bloggers were next. A few days later, another contributor was killed. A keyboard, mouse and sign mentioning the blog were strewn over the corpse.

In a visit to Mexico City earlier this month President Barack Obama focused on economic potential, echoing the upbeat rhetoric of Mexico's new president, Enrique Pena Nieto. Lucy said some good things were indeed happening but that the leaders played down violence and narco-trafficing. "These are primordial issues."