House and Senate cybersecurity systems regularly repel hundreds of thousands of efforts to infiltrate computers and personal devices or infect them with spyware and malware.

But Congress long ago erected a barricade to repel malevolent mail sent to the U.S. Capitol. Authorities began directing mail sent to the Capitol to an off-site location for screening years ago before it ever comes close to the Congressional complex.

Many Congressional offices bar the public from hand-delivering packages. All persons entering the Capitol or Congressional office buildings must first clear security. But those checks don’t inspect for possible chemical, biological or radioactive agents. Hence the reason most offices prefer that closed envelopes and packages are sent outside for checks.

It was just weeks after the September 11 terrorist attacks in 2001. The offices of then-Senate Majority Leader Tom Daschle (D-SD) and Sen. Patrick Leahy (D-VT) received letters sent laced with anthrax spores. It took seven years, but the FBI labeled Bruce Edwards Ivins as the lone culprit in the attacks. Ivins worked for the government at its biodefense lab at Fort Detrick, MD. Ivins committed suicide before facing charges.

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In addition to the 2001 missives sent to Capitol Hill, letters were also addressed to ABC, CBS, NBC, the New York Post and the National Enquirer. Grant Leslie, who worked in the mail office for Daschle, opened the first letter on October 15, 2001. That spurred Congressional officials to halt mail deliveries to the Capitol. The letter addressed to Leahy wasn’t discovered until mid-November and was sent incorrectly to a State Department mail facility in suburban Washington. That’s where a postal worker was exposed to anthrax by inhaling the spores.

The letters sent to Capitol Hill were copied and referenced 9/11. “DEATH TO AMERICA” threatened the letters. “DEATH TO ISRAEL. ALLAH IS GREAT.” The exterior of the envelopes bore slanted, capitalized writing with a fictitious return address. “4th GRADE GREENDALE SCHOOL FRANKLIN PARK NJ 08852.”

The Administration of President George W. Bush attempted to tie the anthrax attack to al-Qaeda. Others suggested Iraq was the culprit. Officials also squeezed government scientist James Hatfill who denied involvement.

But with the anthrax attacks, it turned out that Hatfill wasn’t the real McCoy, formally exonerating him in 2008.

The EPA spent $27 million in Superfund money to fumigate the Hart Senate Office Building with chlorine dioxide gas, the home to Daschle’s office. Hart was closed for weeks. The Ford House Office Building also required decontamination.

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But after the anthrax incident, no mail would ever come to Capitol Hill again without first facing irradiation and inspection someplace else.

Letters laced with ricin wound up in the mail stream in late 2003. One letter was intended for the White House. Another letter connected to the 2003 attack turned up in the mailroom of then-Senate Majority Leader Bill Frist (R-TN) in the Dirksen Senate Office Building in February 2004. Several aides were exposed to ricin and required medical treatment. “Fallen Angel” took credit. But to this day, the FBI has never arrested a suspect.

Capitol Hill found itself again on high alert in April 2013. Authorities intercepted a letter testing positive for ricin at the off-site Congressional screening facility. The letter bound for the Capitol was addressed to Sen. Roger Wicker (R-MS). A letter intended for President Obama surfaced two days later. It also showed signs of ricin contamination.

The letters were mailed from Memphis, TN. “To see a wrong and to not expose it, is to have become a silent partner in its continuance,” read the communiques. The letter closed with the lines “I am KC and I approve this message.”

At first, authorities initially arrested Paul Kevin Curtis from Corinth, MS and accused him of mailing the letters.

When police told Curtis ricin charges, Curtis replied “Rice? I don’t like rice. I don’t eat rice.”

A few days later, police arrested James Everett Dutschke and charged him with the attempted use of a biological weapon. Dutschke was a rival of Curtis. Police believe Dutschke was trying to frame Curtis. Dutschke later pleaded guilty and was sentenced to a quarter century in prison.

There are common denominators in many of these security breaches. The cases have proven challenging to crack or investigators have never definitively solved them. Almost every incident led police down deceptive alleyways and focused on individuals who were suspects or “persons of interest.” Yet many of those leads have proven to be false – often after wrecking the lives of the suspects who were exonerated.

One wonders what’s in store for this investigation.

However, this wave of mail bombs carries a unique signature never observed in the previous attacks: partisanship.

The country was united in 2001 after 9/11. Lawmakers had long abandoned their differences by the time anthrax surfaced in the Senate mail stream a month later. Little was said about parties amid the first ricin threat. The second ricin threat featured letters sent to a Republican and Democrat: Roger Wicker and President Obama.

But things are different now.

Some Democrats immediately blamed the attempted attacks on some of President Trump’s incendiary rhetoric. Reporters observed that Mr. Trump just applauded last year’s incident where Rep. Greg Gianforte (R-MT) bodyslammed reporter Ben Jacobs of The Guardian.

Republicans, in turn, noted it was a supporter of Sen. Bernie Sanders (I-VT) who shot up last year’s Republican Congressional baseball practice, nearly killing Senate Majority Whip Steve Scalise (R-LA).

Some conservatives accused Democrats of launching a “false flag” operation. In the eyes of the extreme right, left-wing activists were responsible for sending the mail bombs in an effort to contour public opinion and paint Democrats as “victims.”

This type of talk represents a dramatic shift from where the nation was after the first batch of “mail” attacks in 2001.

Is this the new normal? There’s a midterm election in a few days. Regardless of which party is in charge of the House and Senate, expect dozens upon dozens of new lawmakers from both sides to arrive on Capitol Hill.

They can either lower the temperature or maintain the heat.