On April 14, I was walking up Boston's Boylston Street to the finish line of the Boston Marathon. After looking at all the emergency trucks, I saw one labeled “Special Operations.” I thought to myself, the marathon finish line is a terrorist target and the authorities know it.

When the bombs went off the next day, I was horrified--but not shocked. But with three killed and about 260 injured, there is a huge amount of human need and that spells opportunity. I’d like to see start-up social enterprises that would make enough profit to finance their operations and growth turn the rest of the profits to charity.

With the unemployment rate high and plenty of college students heading into the summer, there is plenty of talent and energy that could build and operate social enterprises to help out those whose lives have been tragically altered by the Boston bombings.

Here are five start-up ideas that could help Boston - from the lowest to the highest tech.

Services to help injured navigate their new daily lives.

Many of those injured by the Boston bombs lost parts of their legs. They will need help with the kind of daily activities that most of us take for granted--getting up and down stairs, going to doctor’s appointments, and shopping.

To that end, a start-up could provide carefully screened, trained, and monitored people who would help those who have been injured. They could help them with activities from finding contractors to build wheelchair ramps to driving them back and forth from their daily routines.

Assistance paying medical and rehabilitation bills. 

It is highly likely that the injured are incurring medical bills that are not covered by their insurance.

Amputees must pay between $7,200 for a basic below-the-knee prosthetic to $90,000 for a high-tech microprocessor-controlled full leg, according to the Washington Post. It reports that 20 states including Massachusetts require health insurers to pay for part of the cost of prosthetic limbs. Their rehabilitation costs tens of thousands of dollar more.

A start-up could help the injured seek out help to pay these bills. The venture could help each of the injured claim his or her share of the $23 million in individual and corporate donations, health insurance, and hospital charity funds. And it could calculate the amounts still owed and help devise ways - such as setting up charitable funds for each individual--to close the gap.

Career services to help injured people find jobs that tap their skills.

Another big burden facing the injured will be the need for a job to help pay these bills and generate the cash flow for all the other things they need. Some may be able to return to their pre-marathon jobs.

But there may be among the 260 injured--people whose jobs depend on their ability to run or walk quickly. And they may need to find new employment.

A start-up could help them with that challenge. It would help them assess their strengths and weaknesses in different fields, identify the job category that best fits their capabilities, assist in networking with hiring managers, training them to handle interviews, and providing assistance with negotiating employment agreements.

Image search to speed up terrorist identification.

If authorities had been able to pop the images of the Tsarnaev brothers into a search engine and identified them on the day of the bombing, they could have saved the life of the MIT police officer they allegedly murdered three days later.

Instead of identifying them privately and arresting them, authorities released the CCTV videos to the public which tipped off the Tsarnaevs and kicked off the events that included the death of the police officer.

A start-up--staffed with world-class technical talent--could develop a better image search capability. (I have tried Google Image Search and it is very weak.) The venture would allow authorities to take the image of a suspect from a video camera or cell phone, refine it to be useful in a search.

The start-up’s services would compare the refined suspect image to those in government (e.g., driver’s licenses, passport photos, terrorist watch lists), Internet, and social media databases, and produce a short list of top matches that could be examined by human investigators who would narrow down the suspects.

Prosthetics to help injured run the 118th marathon.

Among the 260 injured, there are surely some who lost parts of limbs who would love to dedicate their lives to running in the next marathon. I can’t imagine a more inspiring goal.

And while there is a wide variety of different prosthetics available - including the twin blades on which Oscar Pistorius ran the 2012 Olympics 400-meter race -- it is unclear whether any of them have been designed to withstand the specific challenges of running a marathon.

A venture staffed with world-class talent could develop prosthetics specifically for marathon runners. That technology could be in the works, according to Hugh Herr, who heads the Biomechatronics research group at MIT’s Media Lab.

Each of these five ideas could potentially meet the tremendous human need created by the Boston bombing. But thinking about them raises difficult questions: Who would pay for their services? What would a reasonable price for them to charge? Who would invest in them and why? How would they attract, pay, and motivate the right people? Could they generate sufficient cash flow to sustain themselves?

The Boston bombing has created big unmet needs. I hope these sustainable start-up ideas help you think of better ways to satisfy the needs.