Investigators still do not know why Stephen Paddock shot and killed 58 people during an outdoor music festival in Las Vegas last October, CBS News reports. In a press conference Friday, authorities conceded that three months of investigation had not yielded any findings on Paddock's motivations, though Clark County Sheriff Joe Lombardo did emphasize that Paddock acted alone and that his girlfriend — who was at one point suspected of helping him — would not be charged with any crime.

During the press conference, Lombardo also discussed a newly released 81-page report that examined Paddock's actions in the months leading up to the shooting, which was the deadliest in modern American history. The evidence indicated Paddock had been planning an attack for a while; investigators found he purchased over 50 firearms in the 12 months leading up to the shooting, and that he had studied the response strategies of various law enforcement departments.

Lombardo noted that "disturbing" internet searches Paddock had conducted indicated he may have considered carrying out the attack at other concerts or at beaches in California, CBS News reported. Investigators also found child pornography on Paddock's computer.

Read the full report at the Las Vegas Review Journal. Kelly O'Meara Morales

Ever heard the one about infrastructure week?

As politicians make calls to rectify the decline of U.S. infrastructure investment, a new study from Leah Brooks of George Washington University and Zachary of Liscow of Yale University provides "suggestive evidence" as to how and why infrastructure costs could have changed over time.

After digitizing annual state-level data on interstate highway construction — one of the largest projects in American history — they found that per-mile construction costs increased dramatically over time, tripling between the early 1960s and the 1980s. But why?

Brooks and Liscow ruled out some old theories such as the idea that highway planners procrastinated and left the most geographically challenging routes for last, or that costs for labor and materials changed. Instead, one possible explanation they found is that as incomes and home values rose, so did demand for more expensive interstates. For example, the doubling in real median per capita income between the '60s and '80s accounts for about half the increase in construction costs per mile. States were also building more bridges and ramps as incomes increased. The study suggests all this could have occurred simply because the more money people had, the more willing they were to spend it.

One other factor that the study finds consistent with the data is the rise of the "citizen voice" in the late '60s and early '70s. That includes the growth of environmental activism, the civil rights movement, and homeowners organizing as lobbyists. Basically, more power to the people possibly meant more expensive highway construction. The timing checks out, at least.

The findings, the paper says, are suggestive, but not causal. Still, it looks like a good start in bettering our understanding of infrastructure investment over time. Read the full study here. Tim O'Donnell

House Minority Leader Kevin McCarthy (R-Calif.) on Tuesday insisted that President Trump's tweets telling four minority congresswomen to "go back" to where they came from were not racist.

McCarthy in a press conference was asked whether Trump's weekend tweets were racist, to which he flatly responded, "No," saying that "this is about ideology" and criticizing the so-called "squad" that Trump attacked while not offering a specific defense of the language used. McCarthy also said he will vote against the resolution condemning Trump's tweets, suggesting "individuals on the other side of the isle" have also made comments that should be condemned and insisting that Trump has "clarified" his tweets sufficiently.

Rep. Liz Cheney (R-Wyo.) during this conference also said that Republicans' "opposition to our colleagues' beliefs have absolutely nothing to do with race" while arguing that their policies "would destroy America" and that some of their comments are actually what's racist.

This press conference came shortly after Trump once again insisted on Twitter that his tweets were not racist and that he doesn't "have a racist bone in my body!" Trump shortly after the press conference thanked McCarthy for defending him with a misquote of McCarthy's statement, even as he faced backlash from some Republicans. Watch McCarthy's response below. Brendan Morrow

The New York Police Department officer accused of strangling Eric Garner will not face federal charges.

NYPD officer Daniel Pantaleo will not be charged in the chokehold death of Garner, whose repeated last words of "I can't breathe" became a rallying cry for the Black Lives Matter movement, the Justice Department announced Tuesday. That decision reportedly comes after the DOJ's civil rights division recommended charges, but Attorney General William Barr overruled that suggestion, a senior DOJ official tells ABC News' Alex Mallin.

Garner's July 17, 2014 death was caught on camera after NYPD officers stopped him for allegedly selling cigarettes on a Staten Island street corner. Federal prosecutors then had five years to press charges against Pantaleo, who appeared to use an illegal chokehold on Garner to restrain him and could've been accused of violating Garner's civil rights. But with the statute of limitations on Garner's expiring Wednesday, prosecutors declined to press charges.

That decision, officials tell ABC News and NBC News, comes against the wishes of lawyers in the DOJ's civil rights division. But it's in line with what the U.S. Attorney's Office in the Eastern District of New York recommended, those officials continued.

Pantaleo did face disciplinary action from the NYPD and has been on desk duty without a gun since Garner's death, The New York Times notes. The NYPD also wrapped a disciplinary trial against Pantaleo in June to determine if he should face further punishment. Kathryn Krawczyk

Former Republican congressman Mark Sanford, who lost his primary in 2018 to a Republican opponent Trump endorsed, says he's considering challenging Trump for the 2020 Republican nomination.

Sanford told The Post and Courier on Tuesday that he plans to weigh a possible 2020 run over the next month, saying that "I feel convicted" and that "the Republican Party has lost its way on debt, spending and financial matters." Should he decide not to challenge Trump, Sanford, who specified he would run as a Republican, said he might instead set up a deficit-focused think tank.

Sanford, who as governor of South Carolina disappeared for six days while secretly having an affair even as his staff claimed he was hiking, often criticized Trump while in Congress, in 2017 telling Politico Trump has "fanned the flames of intolerance." Trump in 2018 went after Sanford on Twitter on the day of his Republican primary as "very unhelpful to me in my campaign to MAGA," endorsing his Republican challenger, Katie Arrington. Sanford was ultimately defeated by Arrington, who lost in the general election. After Sanford's primary loss, Trump reportedly described him in a meeting as a "nasty guy." Brendan Morrow

In the not-so-distant past, former Rep. Beto O'Rourke (D-Texas) was the fundraising king. He excelled at reeling in the dough during both his 2018 Texas Senate run and his early presidential campaign. But those days are seemingly over for the 2020 candidate.

O'Rourke has struggled recently when it comes to polls and funding, which is raising questions about whether his once-promising campaign has run out of gas. He is expected to report just $3.6 million between April and June, less than half the $9.4 million he raised in the first quarter. The number also falls short of the $6.1 million he raised in the 24 hours after he first announced his campaign, which is what had people thinking he could be a contender in the first place. Politico called the April through June figure "startlingly small."

The fundraising decline reportedly has O'Rourke's allies on edge, though they think he still has time to get things back on track. If that's to be the case, he probably needs to simultaneously improve his polling numbers, which have also dipped.

It doesn't sound as if O'Rourke is ready to bow out, however. Instead of scaling back, the campaign is making a push by expanding its number of field offices in Iowa.

But in the larger picture, the numbers indicate O'Rourke is fading into the primary's muddied waters. Sens. Elizabeth Warren (D-Mass.), Kamala Harris (D-Calif.), Bernie Sanders (I-Vt.), South Bend Mayor Pete Buttigieg, and former Vice President Joe Biden have begun to separate themselves from the pack in terms of cash and polling data. O'Rourke was never a frontrunner, but he appears to have been displaced by Buttigieg as the election's upstart candidate. Tim O'Donnell

Joe Biden is ready to get civil.

The former vice president is prepared for President Trump to question his age and mental state, just like Trump did during the 2016 presidential race against Hillary Clinton. But instead of challenging Trump to a physical fight like he's mentioned in the past, Biden would rather take Trump on in a push-up contest, he told MSNBC's Mika Brzezinski in a Morning Joe interview aired Tuesday.

Biden and Trump are two objectively old men, yet they've nevertheless publicly fantasized about beating each other up a number of times in the past. So Brzezinski asked Biden what he'd do during a debate if Trump continued to "go after your age, your mental state." "I'd say, 'C'mon Donald, c'mon man. How many push-ups do you want to do here, pal?'" Biden responded. "I mean, jokingly. C'mon, run with me, man." Biden then went on to say he was "not going to get down in the dirt" with Trump, because "that's the only way he knows how to fight."

Perhaps Sen. Chuck Grassley (R-Iowa), nearly 10 years Biden's senior yet famous for his push-up contests with much younger constituents and reporters, would like to get involved. Fellow 2020 candidate Sen. Bernie Sanders (I-Vt.), who's around the same age as Biden and Trump, meanwhile had no comment. Kathryn Krawczyk