Mark Memmott, NPR

(Note at 7:50 a.m. ET, April 18: We've begun a new post to track Thursday's developments.)

Investigators made progress Wednesday, as they tried to determine who planted two bombs near the finish line of the Boston Marathon, leaving three people dead and injuring about 180.

Update at 8:44 p.m. ET. Authorities Make An Arrest:

Authorities have made an arrest in connection to the suspicious envelopes sent to a senator and President Obama.

The FBI identified the suspect as Paul Kevin Curtis.

The The Clarion Ledger reports:

"The wife of a former justice of the peace is being held on a capital murder charge in the killings of the Kaufman County District Attorney, his wife and a top prosecutor," The Dallas Morning News reports.

Margaret Thatcher, the prime minister whose time leading Great Britain in the 1980s brought joy to conservatives and despair to liberals, was remembered Wednesday for "a life lived in the heat of political controversy."

With her death last week at the age of 87, "there is great calm" for the Iron Lady, added the bishop of London, the Right Reverend Richard Chartres, during a funeral service at London's St. Paul's Cathedral.

The day after two bombs exploded near the finish line of the Boston Marathon, investigators began to unravel some of the details of what happened, and we began to learn about the lives of the three people who were killed.

FBI Special Agent in Charge Richard DesLauriers said that they believed the devices used in the attack may have been pressure-cooker bombs stuffed with BBs and nails. Investigators said the bombs may have been left inside nylon bags or backpacks.

Jenna Bush Hager, daughter of former President George W. Bush and former first lady Laura Bush, gave birth over the weekend to a girl.

Margaret Laura "Mila" Hager is named for her grandmothers, the former president announced in a statement. According to The Associated Press, the little girl's nickname is pronounced MEE-luh.

Walking a line meant to show both resolve and willingness to trust in diplomacy, Secretary of State John Kerry warned North Korea on Friday not to engage in more warmongering — but also said the U.S. is willing to talk with that communist state if it's serious about discussing denuclearization.

"No one is going to talk for the sake of talking," Kerry said, but the U.S. does want to see a peaceful resolution of the latest crisis on the Korean peninsula.

As the world waits for what's expected to be another ballistic missile test by North Korea sometime in the next few days, NPR's Frank Langfitt reports there's reason to think that tensions on the Korean Peninsula might soon ease.

We last updated this post at 1:50 p.m. ET:

Saying "this is a start and it's not the end of our work," Democratic Sen. Joe Manchin of West Virginia confirmed Wednesday morning that he and Republican Sen. Pat Toomey of Pennsylvania have put together a bipartisan plan that they believe will "keep guns out of dangerous hands."

"My first thought was I need to go catch him."

Margaret Thatcher, who as British prime minister in the 1980s became known as the "Iron Lady" for her tough economic policies, her partnership with President Reagan in standing up to communism and the short war with Argentina over the Falklands, has died.

Her spokesman, Lord Bell, tells the British Press Association that Baroness Thatcher died Monday following a stroke. She was 87.

A task force launched by the National Rifle Association after the Dec. 14 school shooting in Newtown, Conn., has come back with a report that recommends the creation of programs that give additional weapons training to school resource officers as well as "selected and designated school personnel" who could then carry arms.

Adding some details to an initiative he announced during his latest State of the Union address, President Obama on Tuesday said that federal agencies plan to spend $100 million to jump start an effort to map the human brain. It's research that could lead to breakthroughs in the treatment and prevention of brain disorders.