Menwith Hill is the National Security Agency’s largest overseas surveillance facility, located near the small town of Harrogate in North Yorkshire. As The Intercept revealed in 2015 , the base has been used to aid “a significant number of capture-kill operations” across the Middle East and North Africa, according to top-secret documents. The facility operates spy satellites used to pinpoint the locations of people on the ground below, and it is equipped with eavesdropping technology that can harvest data from more than 300 million emails and phone calls a day.

Questioned in 2013 about GCHQ’s presence at the base, the British government had insisted that it “would not comment on whether there are personnel working in intelligence” there – a position that appears to have changed with Lancaster’s admission, possibly unintentionally. His statement came in response to a Parliament member’s question about how many people are working at Menwith Hill. A spokesperson for the U.K. government’s Ministry of Defence declined to answer questions about whether the statement represented a policy shift.

Earlier this month, British Minister of State for the Armed Forces Mark Lancaster disclosed to the U.K. Parliament that employees of eavesdropping agency Government Communications Headquarters, or GCHQ, are stationed at a remote base in the north of England called Menwith Hill. An unknown number of GCHQ employees are among 578 British civilians, military, and contractors at the site, Lancaster confirmed in a previously unreported written statement , alongside 627 Americans.

The U.K. government is facing fresh calls to clarify its role in U.S. drone strikes after acknowledging that there are potentially hundreds of British spy agency personnel working inside a U.S.-controlled surveillance base that has played a key role in so-called targeted killings.

During the presidency of Barack Obama, covert drone strikes and special operations were the favored method to kill suspected terrorists, particularly in places such as Yemen, Somalia, and Pakistan. The attacks attracted fierce criticism because they may have violated international law, often resulted in civilian casualties, and relied on imprecise methods to identify targets. Under the Trump administration, the same tactics have been pursued even more aggressively. Trump has increased drone strikes and special operations raids, while granting the CIA and the military more autonomy to launch attacks and scrapping constraints intended to prevent civilian deaths.

Jennifer Gibson, a staff attorney with London-based human rights group Reprieve, said lethal U.S. operations have “skyrocketed” since Trump’s inauguration, “killing scores of civilians in countries like Yemen.” She questioned the role of British personnel in the attacks, which have never been approved by or debated in Parliament.

“There are now serious questions over the involvement of U.K. personnel and territory in these [attacks],” Gibson said. “Why, for example, are there hundreds of GCHQ and ‘civilian’ staff at Menwith Hill, a base that plays a key role in the U.S.’s covert assassination program? Ministers need to come clean to Parliament and the public over Britain’s true role in secret U.S. wars.”

One former drone operator previously told The Intercept that the NSA helped locate drone targets by analyzing the activity of a cellphone’s SIM card. “It’s really like we’re targeting a cellphone,” said the former drone operator. “We’re not going after people — we’re going after their phones, in the hopes that the person on the other end of that missile is the bad guy.” The use of such methods was effectively confirmed in 2014 by former CIA and NSA director Michael Hayden, who declared during a debate at Johns Hopkins University: “We kill people based on metadata.”

According to top-secret documents obtained by The Intercept from the whistleblower Edward Snowden, the NSA developed new surveillance programs at Menwith Hill to pinpoint people accessing the internet in remote parts of the world. With code-names such as GHOSTHUNTER and GHOSTWOLF, the programs provided support for conventional British and American military operations in Iraq and Afghanistan. But they also aided covert missions outside declared war zones in countries, such as Yemen, Lebanon, Pakistan, and Somalia. In 2010, the NSA launched a new technique at Menwith Hill to identify targets at internet cafes in Yemen’s Shabwah province and in the country’s capital, Sanaa. The technique was linked to an effort to “capture or eliminate” suspected terrorists in the country, the documents revealed, suggesting it had likely aided the U.S. drone program there.

In the context of English and international law, people who are killed outside of declared war zones are not considered “combatants,” and therefore those responsible for their deaths are not entitled to “combatant immunity” and can potentially be prosecuted. In 2015, a British parliamentary committee said that the lack of clarity about the U.K. government’s policy on U.S. targeted killings “makes accountability difficult” and “potentially exposes front line personnel to criminal liability for the unlawful use of lethal force.”

A spokesperson for the U.K.’s Ministry of Defence told The Intercept that it would not address specific questions about Menwith Hill’s role in U.S. operations. The base “is part of a worldwide U.S. Defence communications network” and supports “a variety of communications activity,” the spokesperson said in an emailed statement. “For operational security reasons and as a matter of policy, neither the MOD nor the DoD publicly discuss specifics concerning military operations or classified communications regardless of unit, platform or asset.”