The American public still does not know how the Obama administration legally justifies its vast killing program. But thanks to years of quietly accumulating political pressure, mounting criticism from important US allies, and a very public airing of widely-held concerns last night, that may soon change.

Europe raises the red flag on the so-called targeted killing program

The United States has reportedly killed 4,700 people in "war on terror" operations outside of declared war zones. On Wednesday, the European Parliament heard a special briefing on the US kill programs from the ACLU's Hina Shamsi and the UN's special rapporteur on counterterrorism and human rights, Ben Emmerson. Following the briefing, the parliamentarians issued a statement calling into question the legality (and morality) of US strikes. The United States cannot hide its legal justification for these operations from the world any longer, they wrote:

"We are deeply concerned about the legal basis, as well as the moral, ethical and human rights implications of the United States' targeted killing programme that authorises the CIA and the military to hunt and kill individuals who have suspected links to terrorism anywhere in the world. "Despite having abandoned the 'war on terror' rhetoric, the US sticks to the notion that it is in the realm of a war, and not organised criminality, when fighting terrorism. It has a destabilising effect on the international legal framework … "There are a growing number of reports demonstrating that hundreds of civilians are being killed in the framework of the targeted killing program. This is being done without any transparency in justification of a 'wartime' policy. We urge our American allies to address the pressing questions over the legal criteria at the basis of a policy that, in targeting so-called militants, destroys both innocent human beings and our common legal heritage." [My emphasis]

The members announced that the European Parliament will hold hearings next month to look further into the US program. Amid such fierce criticisms from key US allies, one has to wonder what the Obama administration is hiding in the legal memos justifying its killing operations. If everything the US is doing is truly above board and complies with international and domestic law, why don't officials release the memos?

What does the president have to hide?

Senator Paul forces the transparency question

While our European allies call for greater transparency and a realignment of the United States' foreign operations to conform with international legal standards, one specific question relating to the killer-drone program appears to have struck a chord with the American public: can the president of the United States authorize the use of lethal force on US soil?

Republican senator for Kentucky, Rand Paul, wants a straightforward answer to this simple question. The Obama administration will not provide one. In a letter dated 20 February 2013 (pdf), the third of its kind, Paul asked soon-to-be CIA Director John Brennan:

"Do you believe that the president has the power to authorize lethal force, such as a drone strike, against a US citizen on US soil, and without trial?"

The senator declared, "I believe the only acceptable answer to this is no," before stressing that if he did not receive a substantive reply, he would "use every procedural option at my disposal to delay your confirmation and bring added scrutiny to this issue and the administration's policies on the use of force". The administration once again ducked the question.

In response to Senator Paul (pdf), Attorney General Eric Holder repeated a line we have heard from John Brennan, as well as from the president himself: the administration has not targeted and does not have plans to target people for death on US soil. Slapping some confusion on top of this evasion, Holder proceeded to sketch out scenarios during which he said he could conceive of the president directing the military to use lethal force domestically.

But the examples he gave – Pearl Harbor and 9/11 – have nothing to do with the simple question Paul asked. The letter is both evasive and dismissive.

Finally, Thursday afternoon, Attorney General Holder sent Senator Paul a 43-word letter:

Of course, we won't know what "engaged in combat" means to the Department of Justice unless it releases the legal memos outlining its justification for the use of lethal force.

The Obama administration may be tired of dealing with these oversight questions and questioners, but those Americans who are paying attention are hardly bored by the underlying constitutional concerns. That was made abundantly clear last night when Senator Paul followed through on his promise to "delay" Brennan's confirmation vote by holding forth for nearly 13 hours on the Senate floor. His filibuster has lit a match to a controversy that has been brewing across the political spectrum for at least the past four years.

Paul's concerns appear to be driven by fears that President Obama (or a future administration) would deploy lethal force in non-emergency situations in the United States. But Paul also hammered the administration on another part of the "targeted" killing debate: the controversial killings that the executive branch refuses to legally or morally justify before the public, among them the strike against a US teenager, Abdulrahman al-Awlaki. Of his death, Paul said:

"There was a man named al-Awlaki. He was a bad guy, by all evidence available to the public that I've read, he was treasonous. I have no sympathy for his death. I still would have tried him in a federal court for treason and I think you could have [had him] executed. But his son was 16 years old, had missed his dad, gone for two years. His son sneaks out of the house and goes to Yemen. His son is then killed by a drone strike. They won't tell us if he was targeted. Suspect, since there were other people in the group, about 20 people killed, that they were targeting someone else. I don't know that. I don't have inside information on that. But I suspect that. "But here's the real problem: when [Robert Gibbs] was asked about al-Awlaki's son, you know what his response was? This I find particularly callous and particularly troubling. The president's [aide's] response to the killing of al-Awlaki's son, he said he should have chosen a more responsible father. "You know, it's kind of hard to choose who your parents are. That's sort of like saying to someone whose father is a thief or a murderer or a rapist, which is obviously a bad thing, but does that mean it's OK to kill their children? Think of the standard we would have if our standard for killing people overseas is, you should have chosen a more responsible parent." [My emphasis]

Paul also railed against the Obama administration's use of so-called "signature strikes":

"We don't have to know what your name is, who you are, who you're with. If you're in a line of traffic and we think you're going from talking to bad people to talking to other bad people, we'll kill you … "The Wall Street Journal says the bulk of the attacks in Pakistan have been signature attacks, meaning: nobody named and nobody specifically identified, and that civilians aren't really counted because anybody, any male between the age of 16 and 50, is a combatant unless otherwise proven. "But if those are the standards, I think we need to be alarmed."

Senator Paul stressed that his concerns were not partisan in nature. The administration wants us to "just trust them", Paul said, but "it's not really about them … It is about the law."

Getting closer to the truth

But now, it is the morning after, and the need to hear the truth is ever more pressing: we still don't have answers to the most basic questions about the lethal powers the Obama administration has claimed. These evasions notwithstanding, Senator Paul hasn't been the only person in Congress to flex some oversight muscle. Some members of the Senate Intelligence Committee threatened to hold up Brennan's confirmation as CIA director until they were given access to the top-secret Justice Department's Office of Legal Counsel memos that contain the administration's legal justification for its kill programs.

As a result, they were granted the opportunity to look at (but not touch!) two more of those 11 memos. (They had briefly been permitted to see two others at the time of Brennan's confirmation hearing last month, but could not share them with their staffs.) And then, they stood down: on 5 March, Brennan's nomination was reported out of their chamber by a 12-3 vote.

The dozen intelligence committee members voted to support the nomination despite the fact that, as Scott Shane reports in the New York Times:

"[T]he administration withheld the opinions governing strikes targeting non-Americans that the committee has also sought, arguing that they are confidential legal advice to the president. As a result, the detailed legal rules for a vast majority of drone strikes, including so-called signature strikes aimed at suspected militants whose names are unknown to the people targeting them, remain secret even from the Congressional intelligence committees." [My emphasis]

Senator Ron Wyden (Democrat, Oregon), who was among the senators demanding to see the memos, told Shane:

"I think this debate is just beginning. The nature of warfare has changed so dramatically. The rules with respect to targeted killing, I think, do need to be made public."

On Wednesday, Attorney General Holder told the Senate judiciary committee that we would "hear from the president in a relatively short period of time" about how the government's kill programs are done "reluctantly", and "in conformity with international law, with domestic law, and with our values as of the American people". But in order to truly understand the president's reasoning, we need to see the memos.

The Senate faced a key test Thursday on whether it would exercise its constitutional role as a check on the power of the presidency. It failed that test by voting to confirm Brennan's nomination by a vote of 63-34. Afterwards, Senator Patrick Leahy (Democrat, Vermont), who ultimately opposed confirming Brennan, explained his vote:

"I have worked with John Brennan, and I respect his record, his experience, and his dedication to public service. But the administration has stonewalled me and the judiciary committee for too long on a reasonable request to review the legal justification for the use of drones in the targeted killing of American citizens. The administration made the relevant OLC memorandum available to the Senate select committee on intelligence in order to advance this nomination. I expect the judiciary committee, which has oversight of the Office of Legal Counsel, to be afforded the same access. For that reason, I reluctantly opposed Mr Brennan's nomination."

In truth, no senator should have been willing to support the Brennan nomination until all of the kill program legal opinions were disclosed to Congress and made public. But there is still hope.

The chairs of both the House and Senate judiciary committees have said they may subpoena the Department of Justice to get them. Wednesday, Senator Leahy told Holder:

"I realize the decision is not entirely in your hands, but [the issue] may be brought to a head with a subpoena from this office, from this committee."

Congressman Bob Goodlatte (Republican, Virginia), chair of the House judiciary committee, has said there is "bipartisan interest" in subpoenaing the executive branch to force it to divulge not only the "targeted" killing memos, but also those that describe the broader powers to kill in "signature strikes". In a letter to the president (pdf) dated 8 February 2013, he and five colleagues wrote:

"We are disappointed that three prior requests to review these memoranda by members of the committee have gone unanswered. We hope that you will affirm your commitment to transparency and openness by accommodating our request to review these documents. We respectfully request that you direct the Justice Department to provide the requested documents to the committee by close of business on Tuesday, 12 February 2013."

The administration did not respond.

The last couple of weeks have been a wake-up call for the United States, and the whole world, on the breathtakingly broad lethal authorities that the Obama administration appears to claim. At the same time, the incredible efforts required to get the Obama administration to disclose, even just to Congress, any of its legal claims should make clear that getting the president to come clean on his legal justifications will require even greater energy and tenacity. Everyone, from senators in Congress to members of the European Parliament, to we ordinary people, will have to be committed to it for the long haul.

We must not let state secrecy obscure critical matters of life and death. The stakes are far too high.

• Correction: a previous version of this article incorrectly listed Ben Emmerson position for the UN as special rapporteur on targeted killings and extrajudicial executions. He is the UN's special rapporteur on counterterrorism and human rights. We apologise for the error